TST: harmonize testing namespace in TestCase (GH8023)
closes #8023 

Merged via https://github.com/pydata/pandas/commit/ace3c46be0ebf12f0eb4b63f22843d76b67ccfde

@papaloizouc Are you interested in trying to add the other `assert_` functions to the `TestCase`?
 @jorisvandenbossche sure will do
 I think we should try to do the adding of the methods a bit more automatically (so also the docstring are kept as the original).

Eg something like `setattr(TestCase, 'assert_frame_equal', staticmethod(assert_frame_equal))` adds the method to the class keeping the docstring and signature

@jreback 
 sure easy enough to inspect the module namespace and auto add the methods to TestCase via a generated wrapper
 Merged via https://github.com/pydata/pandas/commit/ace3c46be0ebf12f0eb4b63f22843d76b67ccfde
 @papaloizouc Thanks!
