[MAINT] restructure the test in folders.

[mathdugre]

• Closes [MAINT] Organize test in subfolders instead of flat list #637

Changes proposed in this pull request:

Checklist (for reviewers)

This section is for the PR reviewer

• PR has an interpretable title with a prefix (e.g. [BUG], [DOC], [ENH], [MAINT])
  Refer to NumPy Development Guide for a full list
• PR links to GitHub issue with mention Closes #XXXX
• Tests pass
• Checks pass

For new features:

• Tests have been added

For bug fixes:

• There is at least one test that would fail under the original bug conditions

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📚 Documentation preview 📚: https://nipoppy--698.org.readthedocs.build/en/698/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
nipoppy/cli.py	100.00% <100.00%> (ø)
nipoppy/workflows/pipeline_store/upload.py	100.00% <ø> (ø)
nipoppy/zenodo_api.py	100.00% <100.00%> (ø)

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[michellewang]

Do you think we should shorten the names of the test files? E.g. test_config_boutiques.py -> test_boutiques.py since it will be in the config directory anyway?

[michellewang]

🧑‍🍳