chore(fw): clean up and extend test_fixtures framework tests

[spencer-tb]

src/ethereum_test_tools/tests/test_filling/test_fixtures.py (link) is quite messy, as its written upon the tests that existed within the initial repo.

These tests can be improved dramatically to distinctly test each solc version and fork, all tests should generate each test type: state_test, blockchain_test and assert that the blockchain version of the state_test fixture matches that of the blockchain_test fixture.

The refactor should make it easy for us add tests for new forks in the future. I.e simply adding a fork name and a fixture file.