test: replace unittest with pytest

[bwanglzu]

What has been changed:

I refactored all tests dependent on unittest, replaced with pytest. Note: no testing behaviour was changed.

Some general things should be further improved:

1. figure out a better way to perform integration tests, especially validate training behaviour.
2. testing model inference involves a lot of model downloading, maybe test from HF inference endpoint?
3. convert tests into real "unittest" and mock responses.

Fast tests all passed (pytest -s -v tests):

============================== 32 passed, 1 skipped, 11 deselected, 4 warnings in 402.78s (0:06:42) ==============================

slow tests all passed (pytest -s -v -m slow tests):

=================================== 11 passed, 33 deselected, 4 warnings in 3311.94s (0:55:11) ===================================

[tomaarsen]

Thanks a bunch for these changes! I'm happy to merge this after some small edits :)