Python unit tests aren't portable

[samsrabin]

There are some things preventing us from running the Python unit tests anywhere other than the NCAR clusters (or at least machines not specified in CIME's config_machines.xml). Aside from being a continuity risk, this means that we can't run them as GitHub workflows.

Unit tests failing on my Mac:

• test_append_user_nl_step2: PermissionError at os.mknod(ufile) (Fix need/checks for inputdata path in subset_data and Python testing #2985)
• test_check_args_outsurfdat_provided: inputdata directory does not exist (Fix need/checks for inputdata path in subset_data and Python testing #2985)
• test_derecho_mpirun: ERROR: Undefined env var 'LMOD_ROOT'. This one probably just needs to be skipped if not on Derecho, but it should throw a warning. (Only run test_derecho_mpirun Python test on Derecho #2987)
• test_inputdata_setup_files_basic: inputdata directory does not exist (Fix need/checks for inputdata path in subset_data and Python testing #2985)
• test_run_phase: PermissionError in SSPCaseFake.create_clone() at os.mknod(os.path.join(newcase, "user_nl_clm")) (Fix need/checks for inputdata path in subset_data and Python testing #2985)