Fix "ambiguous first argument" warning in test suite (#189)

```
test/fiddle/test_pointer.rb:288: warning: ambiguous first argument;
put parentheses or a space even after '-' operator
```

I noticed this in one of my ruby builds:
https://github.com/ruby/ruby/actions/runs/14838247932/job/41654544867?pr=13159#step:8:1818

---------

Co-authored-by: Jean Boussier <[email protected]>
Co-authored-by: Sutou Kouhei <[email protected]>

Author: 3 people

test/fiddle/test_pointer.rb

@@ -281,11 +281,11 @@ def test_null?
 
     def test_size
       Pointer.malloc(4, Fiddle::RUBY_FREE) do |ptr|
-        assert_equal 4, ptr.size
+        assert_equal(4, ptr.size)
       end
-      assert_equal 0, Pointer.new(0).size
-      assert_equal 0, Pointer.new(0).ref.size
-      assert_equal -1, (Pointer.new(0) + 1).size
+      assert_equal(0, Pointer.new(0).size)
+      assert_equal(0, Pointer.new(0).ref.size)
+      assert_equal(-1, (Pointer.new(0) + 1).size)
     end
 
     def test_size=