TST: Enable tests that aren't currently running in indexes/datetimes/test_tools.py
- [X] closes #17403
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Enabled tests that aren't currently running.  Made a small modification to a typo in `datetimes.py` to make sure all tests pass.  Small modifications to checks in `TestToDatetimeUnit::test_frame` to verify that the raised messages match expectations.

