Add Python system tests of subset_data

For one of many possible examples: Check that the correct lines are written to `shell_commands`.

[`test_unit_subset_data.py`](https://github.com/ESCOMP/CTSM/blob/master/python/ctsm/test/test_unit_subset_data.py) doesn't actually set up a case; it just uses `get_parser()`, `setup_files()`, and `check_args()`.