Run Fortran (pfunit-based) unit tests on each PR

As part of the testing workflow, we should run the Fortran (pfunit-based) unit tests on each PR. Ideally, most or all of this repository would be covered by unit tests, and the unit testing together with the existing scripts_regression_tests would be sufficient coverage for a CESM_share PR to be merged.

See https://github.com/ESCOMP/CESM_share/pull/17#pullrequestreview-792177008 for the current process, though #18 suggests how we could make this more straightforward – but I think from an automated testing perspective it might not make much difference.

Note that this will require downloading & building pfunit as part of the workflow (probably cached like we cache netcdf, etc.).