Make it easier to run the pfunit-based unit tests

Currently, to run the Fortran (pfunit-based) unit tests in this repository, I think you need to clone cime then checkout this CESM_share repository as an external of cime, then run all of the unit tests defined at the top level of cime. (See https://github.com/ESCOMP/CESM_share/pull/17#pullrequestreview-792177008 .) This is an awkward workflow, particularly since I feel like this CESM_share repository is the most important place to increase our unit test coverage, so it should be easy to write and run unit tests from here.

We do still need CIME to run the unit tests, but I think what would be better would be:
- Move the relevant pieces from cime's top-level CMakeLists.txt file into this repository
- The workflow would then be:
- Checkout cime to anywhere on disk
- From the top level of this CESM_share repository (or wherever the top-level CMakeLists.txt file will be defined), run `/path/to/cime/scripts/fortran_unit_testing/run_tests.py --build-dir unit_tests_temp`
- We can probably then delete cime's top-level CMakeLists.txt file, because I think the mct/cpl7 unit tests are no longer important to keep, and I don't think there are any other unit tests run from there (but we should check that)
- We can then also remove cime's scripts_regression_tests test, `test_b_cime_f90_unit_tests`, since it won't have anything to do anymore (but we should keep `test_a_unit_test`, which tests the general unit test infrastructure as defined in CIME).