parametrize more tests, move to test_liboffstes
Some tests currently in test_offsets belong in test_liboffsets, this moves them.

There is a test_shift_month in test_offsets and another in test_liboffsets.  This combines them.

Continuing the process of making the big lists of tests in test_offsets use pytest.mark.parametrize.  Is there any downside to doing this?  e.g. when it collects the tests, will it hold them all in memory (instead of as a generator)?

- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`