MAINT: Remove vestigial self.assert*
Remove all remaining `self.assert*` method calls originating from `unittest`.  Any that are left are calls to methods directly defined in the test class or a higher derived `pandas` test class.

Partially addresses #15990.

Once this PR is merged, it is important that we remain vigilant about requiring PR's to adhere to the `pytest` idiom until we remove our dependency on `unittest.TestCase` (after which the builds will do the work when tests fail).