BUG: assert_index_equal does not raise error for check_categorical=False when comparing 2 CategoricalIndex objects
- [x] closes #19776 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

* tests added for `check_categorical` parameter in `assert_series_equal`
* cleaned up `assert_categorical_equal` parameter order and docstring to match other functions (`obj` at the end)

Thanks to all maintainers who helped answer questions on Gitter during the informal PyCon sprint!