Regroup tests under folder structure (#1443)

### Before

```
test
├── lib
│   └── helper.rb
└── rdoc
    ├── binary.dat
    ├── helper.rb
    ├── hidden.zip.txt
    ├── MarkdownTest_1.0.3
    │   ├── Amps and angle encoding.text
    │   ├── Auto links.text
    │   ├── Backslash escapes.text
    │   ├── Blockquotes with code blocks.text
    │   ├── Code Blocks.text
    │   ├── Code Spans.text
    │   ├── Hard-wrapped paragraphs with list-like lines.text
    │   ├── Horizontal rules.text
    │   ├── Inline HTML (Advanced).text
    │   ├── Inline HTML (Simple).text
    │   ├── Inline HTML comments.text
    │   ├── Links, inline style.text
    │   ├── Links, reference style.text
    │   ├── Links, shortcut references.text
    │   ├── Literal quotes in titles.text
    │   ├── Markdown Documentation - Basics.text
    │   ├── Markdown Documentation - Syntax.text
    │   ├── Nested blockquotes.text
    │   ├── Ordered and unordered lists.text
    │   ├── Strong and em together.text
    │   ├── Tabs.text
    │   └── Tidiness.text
    ├── rdoc_alias_test.rb
    ├── rdoc_any_method_test.rb
    ├── rdoc_attr_test.rb
    ├── rdoc_class_module_test.rb
    ├── rdoc_code_object_test.rb
    ├── rdoc_comment_test.rb
    ├── rdoc_constant_test.rb
    ├── rdoc_context_section_test.rb
    ├── rdoc_context_test.rb
    ├── rdoc_cross_reference_test.rb
    ├── rdoc_encoding_test.rb
    ├── rdoc_extend_test.rb
    ├── rdoc_generator_darkfish_test.rb
    ├── rdoc_generator_json_index_searcher_test.rb
    ├── rdoc_generator_json_index_test.rb
    ├── rdoc_generator_markup_test.rb
    ├── rdoc_generator_pot_po_entry_test.rb
    ├── rdoc_generator_pot_po_test.rb
    ├── rdoc_generator_pot_test.rb
    ├── rdoc_generator_ri_test.rb
    ├── rdoc_i18n_locale_test.rb
    ├── rdoc_i18n_text_test.rb
    ├── rdoc_include_test.rb
    ├── rdoc_markdown_test_test.rb
    ├── rdoc_markdown_test.rb
    ├── rdoc_markup_attribute_manager_test.rb
    ├── rdoc_markup_attributes_test.rb
    ├── rdoc_markup_document_test.rb
    ├── rdoc_markup_formatter_test.rb
    ├── rdoc_markup_hard_break_test.rb
    ├── rdoc_markup_heading_test.rb
    ├── rdoc_markup_include_test.rb
    ├── rdoc_markup_indented_paragraph_test.rb
    ├── rdoc_markup_paragraph_test.rb
    ├── rdoc_markup_parser_test.rb
    ├── rdoc_markup_pre_process_test.rb
    ├── rdoc_markup_raw_test.rb
    ├── rdoc_markup_test.rb
    ├── rdoc_markup_to_ansi_test.rb
    ├── rdoc_markup_to_bs_test.rb
    ├── rdoc_markup_to_html_crossref_test.rb
    ├── rdoc_markup_to_html_snippet_test.rb
    ├── rdoc_markup_to_html_test.rb
    ├── rdoc_markup_to_joined_paragraph_test.rb
    ├── rdoc_markup_to_label_test.rb
    ├── rdoc_markup_to_markdown_test.rb
    ├── rdoc_markup_to_rdoc_test.rb
    ├── rdoc_markup_to_table_of_contents_test.rb
    ├── rdoc_markup_to_tt_only_test.rb
    ├── rdoc_markup_verbatim_test.rb
    ├── rdoc_method_attr_test.rb
    ├── rdoc_normal_class_test.rb
    ├── rdoc_normal_module_test.rb
    ├── rdoc_options_test.rb
    ├── rdoc_parser_c_test.rb
    ├── rdoc_parser_changelog_test.rb
    ├── rdoc_parser_markdown_test.rb
    ├── rdoc_parser_prism_ruby_test.rb
    ├── rdoc_parser_rd_test.rb
    ├── rdoc_parser_ruby_test.rb
    ├── rdoc_parser_simple_test.rb
    ├── rdoc_parser_test.rb
    ├── rdoc_rd_block_parser_test.rb
    ├── rdoc_rd_inline_parser_test.rb
    ├── rdoc_rd_inline_test.rb
    ├── rdoc_rd_test.rb
    ├── rdoc_rdoc_test.rb
    ├── rdoc_require_test.rb
    ├── rdoc_ri_driver_test.rb
    ├── rdoc_ri_paths_test.rb
    ├── rdoc_rubygems_hook_test.rb
    ├── rdoc_servlet_test.rb
    ├── rdoc_single_class_test.rb
    ├── rdoc_stats_test.rb
    ├── rdoc_store_test.rb
    ├── rdoc_task_test.rb
    ├── rdoc_text_test.rb
    ├── rdoc_token_stream_test.rb
    ├── rdoc_tom_doc_test.rb
    ├── rdoc_top_level_test.rb
    ├── README
    ├── support
    │   ├── formatter_test_case.rb
    │   ├── test_case.rb
    │   └── text_formatter_test_case.rb
    ├── test.ja.largedoc
    ├── test.ja.rdoc
    ├── test.ja.txt
    ├── test.txt
    ├── xref_data.rb
    └── xref_test_case.rb
```

```
Finished in 5.179508 seconds.
--------------------------------------------------------------------------------------------------------------
2537 tests, 6688 assertions, 0 failures, 0 errors, 0 pendings, 6 omissions, 0 notifications
100% passed
--------------------------------------------------------------------------------------------------------------
489.81 tests/s, 1291.24 assertions/s

Finished in 0.194474 seconds.
--------------------------------------------------------------------------------------------------------------
20 tests, 46 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed
--------------------------------------------------------------------------------------------------------------
```

### After

```
test
├── lib
│   └── helper.rb
└── rdoc
    ├── binary.dat
    ├── code_object
    │   ├── alias_test.rb
    │   ├── any_method_test.rb
    │   ├── attr_test.rb
    │   ├── class_module_test.rb
    │   ├── code_object_test.rb
    │   ├── constant_test.rb
    │   ├── extend_test.rb
    │   ├── include_test.rb
    │   ├── method_attr_test.rb
    │   ├── normal_class_test.rb
    │   ├── normal_module_test.rb
    │   └── single_class_test.rb
    ├── generator
    │   ├── darkfish_test.rb
    │   ├── json_index
    │   │   └── searcher_test.rb
    │   ├── json_index_test.rb
    │   ├── markup_test.rb
    │   ├── pot
    │   │   ├── po_entry_test.rb
    │   │   └── po_test.rb
    │   ├── pot_test.rb
    │   └── ri_test.rb
    ├── helper.rb
    ├── hidden.zip.txt
    ├── MarkdownTest_1.0.3
    │   ├── Amps and angle encoding.text
    │   ├── Auto links.text
    │   ├── Backslash escapes.text
    │   ├── Blockquotes with code blocks.text
    │   ├── Code Blocks.text
    │   ├── Code Spans.text
    │   ├── Hard-wrapped paragraphs with list-like lines.text
    │   ├── Horizontal rules.text
    │   ├── Inline HTML (Advanced).text
    │   ├── Inline HTML (Simple).text
    │   ├── Inline HTML comments.text
    │   ├── Links, inline style.text
    │   ├── Links, reference style.text
    │   ├── Links, shortcut references.text
    │   ├── Literal quotes in titles.text
    │   ├── Markdown Documentation - Basics.text
    │   ├── Markdown Documentation - Syntax.text
    │   ├── Nested blockquotes.text
    │   ├── Ordered and unordered lists.text
    │   ├── Strong and em together.text
    │   ├── Tabs.text
    │   └── Tidiness.text
    ├── markup
    │   ├── attribute_manager_test.rb
    │   ├── attributes_test.rb
    │   ├── document_test.rb
    │   ├── formatter_test.rb
    │   ├── hard_break_test.rb
    │   ├── heading_test.rb
    │   ├── include_test.rb
    │   ├── indented_paragraph_test.rb
    │   ├── markup_test.rb
    │   ├── paragraph_test.rb
    │   ├── parser_test.rb
    │   ├── pre_process_test.rb
    │   ├── raw_test.rb
    │   ├── to_ansi_test.rb
    │   ├── to_bs_test.rb
    │   ├── to_html_crossref_test.rb
    │   ├── to_html_snippet_test.rb
    │   ├── to_html_test.rb
    │   ├── to_joined_paragraph_test.rb
    │   ├── to_label_test.rb
    │   ├── to_markdown_test.rb
    │   ├── to_rdoc_test.rb
    │   ├── to_table_of_contents_test.rb
    │   ├── to_tt_only_test.rb
    │   └── verbatim_test.rb
    ├── parser
    │   ├── c_test.rb
    │   ├── changelog_test.rb
    │   ├── markdown_test.rb
    │   ├── parser_test.rb
    │   ├── prism_ruby_test.rb
    │   ├── rd_test.rb
    │   ├── ruby_test.rb
    │   └── simple_test.rb
    ├── rd
    │   ├── block_parser_test.rb
    │   ├── inline_parser_test.rb
    │   ├── inline_test.rb
    │   └── rd_test.rb
    ├── rdoc_comment_test.rb
    ├── rdoc_context_section_test.rb
    ├── rdoc_context_test.rb
    ├── rdoc_cross_reference_test.rb
    ├── rdoc_encoding_test.rb
    ├── rdoc_i18n_locale_test.rb
    ├── rdoc_i18n_text_test.rb
    ├── rdoc_markdown_test_test.rb
    ├── rdoc_markdown_test.rb
    ├── rdoc_options_test.rb
    ├── rdoc_rdoc_test.rb
    ├── rdoc_require_test.rb
    ├── rdoc_rubygems_hook_test.rb
    ├── rdoc_servlet_test.rb
    ├── rdoc_stats_test.rb
    ├── rdoc_store_test.rb
    ├── rdoc_task_test.rb
    ├── rdoc_text_test.rb
    ├── rdoc_token_stream_test.rb
    ├── rdoc_tom_doc_test.rb
    ├── rdoc_top_level_test.rb
    ├── README
    ├── ri
    │   ├── driver_test.rb
    │   └── paths_test.rb
    ├── support
    │   ├── formatter_test_case.rb
    │   ├── test_case.rb
    │   └── text_formatter_test_case.rb
    ├── test.ja.largedoc
    ├── test.ja.rdoc
    ├── test.ja.txt
    ├── test.txt
    ├── xref_data.rb
    └── xref_test_case.rb
```

```
--------------------------------------------------------------------------------------------------------------
2537 tests, 6688 assertions, 0 failures, 0 errors, 0 pendings, 6 omissions, 0 notifications
100% passed
--------------------------------------------------------------------------------------------------------------
520.67 tests/s, 1372.58 assertions/s

Finished in 0.185438 seconds.
--------------------------------------------------------------------------------------------------------------
20 tests, 46 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed
--------------------------------------------------------------------------------------------------------------
```

Author: st0012

test/rdoc/rdoc_alias_test.rb renamed to test/rdoc/code_object/alias_test.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require_relative 'xref_test_case'
+require_relative '../xref_test_case'
 
 class RDocAliasTest < XrefTestCase
 

test/rdoc/rdoc_any_method_test.rb renamed to test/rdoc/code_object/any_method_test.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require_relative 'xref_test_case'
+require_relative '../xref_test_case'
 
 class RDocAnyMethodTest < XrefTestCase
 

test/rdoc/rdoc_attr_test.rb renamed to test/rdoc/code_object/attr_test.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require_relative 'helper'
+require_relative '../helper'
 
 class RDocAttrTest < RDoc::TestCase
 

test/rdoc/rdoc_class_module_test.rb renamed to test/rdoc/code_object/class_module_test.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require_relative 'xref_test_case'
+require_relative '../xref_test_case'
 
 class RDocClassModuleTest < XrefTestCase
 

test/rdoc/rdoc_code_object_test.rb renamed to test/rdoc/code_object/code_object_test.rb

@@ -1,7 +1,7 @@
 # coding: US-ASCII
 # frozen_string_literal: true
 
-require_relative 'xref_test_case'
+require_relative '../xref_test_case'
 
 class RDocCodeObjectTest < XrefTestCase
 

test/rdoc/rdoc_constant_test.rb renamed to test/rdoc/code_object/constant_test.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require_relative 'xref_test_case'
+require_relative '../xref_test_case'
 
 class RDocConstantTest < XrefTestCase
 

test/rdoc/rdoc_extend_test.rb renamed to test/rdoc/code_object/extend_test.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require_relative 'xref_test_case'
+require_relative '../xref_test_case'
 
 class RDocExtendTest < XrefTestCase
 

test/rdoc/rdoc_include_test.rb renamed to test/rdoc/code_object/include_test.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require_relative 'xref_test_case'
+require_relative '../xref_test_case'
 
 class RDocIncludeTest < XrefTestCase
 

test/rdoc/rdoc_method_attr_test.rb renamed to test/rdoc/code_object/method_attr_test.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require_relative 'xref_test_case'
+require_relative '../xref_test_case'
 
 class RDocMethodAttrTest < XrefTestCase
 

test/rdoc/rdoc_normal_class_test.rb renamed to test/rdoc/code_object/normal_class_test.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require_relative 'xref_test_case'
+require_relative '../xref_test_case'
 
 class RDocNormalClassTest < XrefTestCase