What kind of change does this PR introduce?

This migrates all the tests from the unittest paradigm to the pytest paradigm:

• Use pytest fixtures instead of setUp/tearDown. This allowed a bit of factorization.
• Use test functions instead of TestCase classes.
• Long test files have been split.
• Django and Flask authorization server config loading part has been moved to dedicated load_config methods.

This makes the unit tests a lot more legible, and a bit shorter. There are still 748 tests as before.

Follow-up to #746

Checklist

• The commits follow the conventional commits specification.
• You ran the linters with prek.
• You reached 100% of code coverage on the code you edited, without abusive use of pragma: no cover

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