TST: add cases for nearest_workday and before_nearest_workday
- [ ] xref #38139
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Add cases for testing observance rules ``nearest_workday`` and ``before_nearest_workday`` for their full coverage.