Python system tests aren't portable

There are some things preventing us from running the Python system 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.

Solving these issues might be unnecessary once we have a container where we can run the tests, as long as we can get that working at least in GitHub workflows (ideally also on personal computers). Not sure when that will be, though.