Address `@xfail_if_external_scheduler` TODOs in `test_model`

[sarahyurick]

Several of our pytests in tests/integration/test_model.py have this note:

# TODO - many ML tests fail on clusters without sklearn - can we avoid this?
@skip_if_external_scheduler

We should look into removing @skip_if_external_scheduler from these pytests.

[sarahyurick]

See also in tests/unit/test_ml_wrappers.py

[sarahyurick]

Now @xfail_if_external_scheduler