Improve Python system/unit tests to enable running anywhere

[samsrabin]

There are some things preventing us from running the Python system and 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. This issue will serve as a parent for some thoughts I've had but haven't necessarily yet investigated.

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 unit 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)

These system tests fail without nco loaded, which complicates testing, especially on systems without it installed (#2988):

• TestSysMeshMaskModifier.test_allInfo
• TestSysMeshMaskModifier.test_modifyMesh

The following system tests fail on my Mac (with nco loaded), but I don't have time at the moment to investigate them. A quick look suggests they're probably all due to my machine's name not being in CIME's config_machines.xml file. If that's the case, I think two changes should be made: (1) add a entry in that file for GitHub runners and (2) catch those errors in our tests and throw a warning instead if no machine is found.

• TestSysMeshMaskModifier.test_allInfo
• TestSysMeshMaskModifier.test_modifyMesh
• TestSysRunTower.test_ad_site
• TestSysRunTower.test_one_site
• TestSysRunTower.test_overwrite
• TestSysRunTower.test_plumber_site
• TestSysRunTower.test_setup_only
• TestSysRunTower.test_xmlchange