fix(pytest): actually check the fork before issuing a fork mismatch warning for yul compilation

[danceratopz]

🗒️ Description

This PR fixes an error introduced in #418: src/pytest_plugins/test_filler/test_filler.py always issued a warning for an incorrect fork, regardless of whether get_closest_fork_with_solc_support() returned the requried fork, or not. For example,

fill tests/homestead/yul/test_yul_example.py --fork=Shanghai -v

with a solc that supports Shanghai incorrectly warns:

============================================= warnings summary ==============================================
tests/homestead/yul/test_yul_example.py::test_yul[fork_Shanghai-blockchain_test]
tests/homestead/yul/test_yul_example.py::test_yul[fork_Shanghai-blockchain_test_hive]
tests/homestead/yul/test_yul_example.py::test_yul[fork_Shanghai-state_test]
  /home/dtopz/code/github/danceratopz/execution-spec-tests/src/pytest_plugins/test_filler/test_filler.py:376:
 UserWarning: Compiling Yul for Shanghai, not Shanghai.
    warnings.warn(f"Compiling Yul for {solc_target_fork}, not {fork}.")

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
======================================= 3 passed, 3 warnings in 0.34s =======================================

🔗 Related Issues

None.

✅ Checklist

• All: Set appropriate labels for the changes.
• All: Considered squashing commits to improve commit history.
• All: Added an entry to CHANGELOG.md. Unreleased; skipping changelog.
• All: Considered updating the online docs in the ./docs/ directory.
• Tests: Included the type and version of evm t8n tool used to locally execute test cases: e.g., ref with commit hash or geth 1.13.1-stable-3f40e65.
• Tests: Ran mkdocs serve locally and verified the auto-generated docs for new tests in the Test Case Reference are correctly formatted.

[spencer-tb]

LGTM