improve test for timestamp addition/subtraction
This PR adds regression test cases for the addition/subtraction of time deltas to timestamps. In fact, this is causing issues in 0.24.2 when `freq="M"` and `freq="W"`, and the problem is solved in `master`.

- [ ] closes #27182 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
