Make sure temporary directories are cleaned up in tests

[marcuscollins]

This got flagged by code rabbit in PR #274

# Must be set before any `import protpardelle...` happens. Respect an
# externally configured directory (e.g. when real weights are available).
os.environ.setdefault(
    "PROTPARDELLE_MODEL_PARAMS", tempfile.mkdtemp(prefix="protpardelle_model_params_")
)

and again in test_protpardelle_wrapper.py.

To resolve this issue, make any necessary changes to make sure the temp directories are cleaned up after tests are completed. If no changes are needed, just add that to the comments that the directories are cleaned up, and how that is known (something in pytest, somewhere else in the code) just so this doesn't get flagged again.