REF: Base class for all extension tests
Added the staticmethods

- assert_series_equal
- assert_frame_equal

to the base class. Useful for test cases like DecimalArray with NaNs,
since our assert_*_equal methods don't properly handle Decimal NaNs.

- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

