BUG: Tick + np.timedelta64
- [ ] closes #xxxx
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

Looks like a pass to de-duplicate the test_business_day and test_custom_business_day files would be worthwhile