Safer rundir cleaning up in test suite

[nbren12]

The test suite manually cleans up the run directory code: https://github.com/VulcanClimateModeling/fv3gfs-fortran/blob/201f4cc6da6a90a477e59d04f58f05f503317e6c/tests/pytest/test_regression.py#L96

This is a bit concerning, since it could be easy to make a change to get_run_dir that e.g. returns /, which could result in disaster.

pytest offers two fixtures (tmpdir and tmpdir_factory) for creating temporary directories that are automatically cleaned up. Was there an intentional decision not to use these in favor of the more explicit but dangerous current approach?

[nbren12]

I noticed this when reviewing #212

[nbren12]

Here's how we use tmpdir_factory for regression testing in fv3net: https://github.com/VulcanClimateModeling/fv3net/blob/29f4d25714db8cda6cb90a00a14e3ce3a5c221e6/workflows/prognostic_c48_run/tests/test_regression.py#L523