Use `self.assertEqual` instead of `assert` in tests.
IMO the test should use `self.assertEqual` instead of `assert` here:

https://github.com/huggingface/transformers/blob/8d171628fe84bdf92ee40b5375d7265278180f14/tests/test_tokenization_deberta_v2.py#L105-L106

I can provide a PR...
> I can provide a PR...

see #11935