Use pytest instead of unittest in run_ctsm_py_tests

[samsrabin]

Description of changes

This PR changes run_ctsm_py_tests to use pytest instead of unittest, enabling many modern features. One test file, test_unit_longitude.py, has been modified to use pytest's "parametrize" functionality as an example. Most of the changed lines in that file are just whitespace differences.

Existing run_ctsm_py_tests arguments have been ported to replicate the same functionality in pytest. Unknown arguments will be passed to pytest directly.

Also: Made it so that the tests can be called from anywhere in the checkout, instead of only python/.

Specific notes

Contributors other than yourself, if any: None

CTSM Issues Fixed:

• Resolves Convert Python unit/system tests from unittest to pytest #3616

Are answers expected to change (and if so in what way)? No

Any User Interface Changes (namelist or namelist defaults changes)? No

Does this create a need to change or add documentation? Did you do so? No

Testing performed, if any: Python unit and system tests pass. New version finds the same number of tests as before.