feat(pytest): add option to write one fixture per json file

[danceratopz]

🗒️ Description

• Adds a command-line option --single-fixture-per-file:

    --single-fixture-per-file
                          Don't group fixtures in JSON files by test function;
                          write each fixture to its own file. This can be used to
                          increase the granularity of --verify-fixtures.
  

• Adds tests for fixture output for different command-line option combinations:

  test_fixture_output_based_on_command_line_args[default-args]
  test_fixture_output_based_on_command_line_args[flat-output]
  test_fixture_output_based_on_command_line_args[flat-output_custom-output-dir]
  test_fixture_output_based_on_command_line_args[flat-output_custom-output-dir_enable-hive]
  test_fixture_output_based_on_command_line_args[single-fixture-per-file]
  test_fixture_output_based_on_command_line_args[single-fixture-per-file_custom_output_dir]
  test_fixture_output_based_on_command_line_args[single-fixture-per-file_custom_output_dir_enable-hive]
  test_fixture_output_based_on_command_line_args[flat-single-per-file_flat-output]
  

🔗 Related Issues

#185

✅ Checklist

• All: Set appropriate labels for the changes.
• All: Considered squashing commits to improve commit history.
• All: Added an entry to CHANGELOG.md.
• 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.

[marioevz]

Looks amazing, I really appreciate the tests under test_test_filler.py, it's really cool how we can test this functionality.