TST: add test for rolling max with DatetimeIndex
- [x] closes #21096
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

thanks @ganevgv 

I would love if you could parametrize some of these tests (e.g. also test min) in a followup if you can. Thanks for the feedback @jreback, @WillAyd - created a new PR using parametrize over different operations and frequencies - #29932