Set up GitHub Actions CI testing

[alperaltuntas]

We need to set up GitHub actions for CI testing. Ideally, we'd like to implement the following checks:

• Test that standalone MOM6 build script works.
• A few lightweight standalone tests to run, e.g., standalone/examples/single_column/KPP
• Run the check_default_params.py test under tests.
• Black check for python scripts under cime_config. (See https://black.readthedocs.io/en/stable/integrations/github_actions.html)

There are two existing GitHub actions examples that can guide this task.

(1) https://github.com/ESMCI/visualCaseGen: Clones a version CESM including visualCaseGen (cesm2_3_beta17_gui) and run CI tests under the visualCaseGen/tests directory.

(2) https://github.com/NCAR/MOM6