Add SystemTest that runs Python system testing

As @slevis-lmwg pointed out during review of #3286, there are some (pre-existing and new) Python system tests that will require updating when we update our default input files. Because that's not an intuitive connection, it'd be easy to commit such breaking changes without remembering to update the tests. It would thus be good to have a SystemTest running the Python tests as part of `aux_clm` for every tag.