Add GitHub workflow for Python unit tests

[samsrabin]

Description of changes

Adds a GitHub workflow job that runs our Python unit tests whenever a relevant file is changed. Required some changes to unit tests to ensure portability, including one being changed to a system test—we'll deal with making that portable later.

Specific notes

Contributors other than yourself, if any: None

CTSM Issues Fixed:

• Resolves #2985
• Resolves #2986
• Resolves #2984

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: As of ac3b0a5b7:

• Python unit tests pass on my Mac
• New GitHub Workflow to run Python unit tests
• Python unit and system testing on Derecho
• [ ] WAITING until after b4b-dev merge 2025-06-10 clm_pymods suite

[rgknox]

totally awesome feature @samsrabin