TST/ERR: Add Period ops tests / fix error message
- [x] related to #13242

closes #13251
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

This must be after #13079.

merged #13079 
see my comments on #13251 (may already be covered here)
 thanks @sinhrks all of these tests are really helpful!
