docs: add tests types to CONTRIBUTING.md

[masci]

Related Issues

• related to Revisit tests #2811

Proposed Changes:

Add to the dev docs the outcome of #2811
Ideally, newly added tests will follow those requirements while we progressively adapt the existing tests to the new model.

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added tests that demonstrate the correct behavior of the change
• I've used the conventional commit convention for my PR title
• I documented my code
• I ran pre-commit hooks and fixed any issue

[ZanSara]

Looks good! I wonder, though, if we should translate these categories into some more practical guidelines for contributors, for example about using the @pytest.integration marker, or creating a new (for now empty) folder for e2e tests where contributors can add their large tests already. Or at least a hint about why there's no match in the actual tests folder for these new groups (something like NOTE: migration of the old tests into these new categories is in progress. Please ask the maintainers if you are in doubt about how to classify your tests or where to place them.)

[ZanSara]

LGTM! 👍