Don't run CTSM during Python tests

There are some `run_tower` Python system tests that actually run CTSM. This harms portability. Instead, these should be run through the CIME SystemTest infrastructure.

- `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`

Probably one of these should be added to the `aux_clm` suite, and all should be added to the `clm_pymods` suite.

Work on this has started on PR #2345, but it's not working yet.