TST: add test for agg on ordered categorical cols
- [x] closes #27800
- [x] tests added / passed
- [x] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

> this is adding a lot of code, can you simplify / parameterize things.

Sure. Planning to add a fixture for the input dataframe and use it across the six test cases.  I feel parameterizing the tests will have a big decorator function on top which might become hard to understand/maintain. Let me know your thoughts. thanks @mathurk1 very nice