CI: mom6 tests on PR's

[rem1776]

Description
Adds a workflow to run the mom6 unit tests on any incoming pull requests. Pulls from the default noaa-gfdl/mom6 branch (which is dev/gfdl).

How Has This Been Tested?
CI

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes

[thomas-robinson]

@marshallward can you make sure this looks right?

[marshallward]

This all looks good to me. Thanks for integrating this into the FMS build.