Minor tweaks to the test harness

[anvacaru]

1. Fixing a bug in Makefile introduced Disable unsupported conformance test-suites #2817. In pytest, the -k flags are additive. I replaced them with a single -k flag that uses and to filter the test suites that get executed.
2. Some tests in test/failing.llvm were failing because they needed a chainId of 1 instead of 0.
3. Added a fallback rule for load "transaction" that captures blocks that are incorrectly encoded.
4. Made adjustments to the _test function by removing redundant rewrites and lookups.
5. Refactored the _skipped_tests function to use frozenset instead of list.

[juliankuners]

lgtm, though I left a small comment