Restored backwards compatibility after refs #27991.
Regression in be6ca893.

Reduces the possibility of circular imports.

SimpleTestCase.databases makes it possible to determine the set of
databases required to run the discovered tests.

Follow up to 8eae0946.

Thanks Nick Pope, Mariusz Felisiak, and Tim Graham for reviews.

Used more specific modules to reduce the risk of circular imports.

Co-Authored-By: Tim Graham <timograham@gmail.com>

Thanks Tom Forbes for the report.

Thanks Mariusz Felisiak for auditing.

There's a bug that causes a test failure in forms_tests:
https://github.com/python-pillow/Pillow/pull/3501/files#r244651761.

Thanks andreage for the report and suggested fix.