Implement missing tests, fix offset floordiv, rfloordiv, fix return types
This splits off of #19365 a bunch of peripheral changes that were never intended to be part of #19365 in the first place.

- Implements many tests in the "how is there not already a test for that!?" category
- Fixes(+tests) Timedelta // DateOffset and DateOffset // Timedelta
- Fixes(+tests) return types for `Timedelta.__add__, __radd__, __sub__, __rsub__(np.timedelta64)` (ATM returns `timedelta64`) 
- Fixes a typo in test_construction that causes a dummy assertion to be made instead of the intended assertion.

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