TST: GH23452 test reorder_categories() on categorical index
- [x] closes #23452
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Couldn't see entries for tests in whatsnew files so I assume it's not needed if only a test case is added This is my first Pandas contribution, looking forward to your feedback.