Update to use more test fixtures

[jeanconn]

Description

Update to use more test fixtures. This moves the agasc supplement disable to conftest.py and makes the shared data in test_find_er_catalog use fixtures that also respect the CHANDRA_MODELS_DEFAULT_VERSION pinned in conftest.py.

This finishes #190 . Without the changes to test_find_er_catalog, the ska_testr sparkles test will continue to include a UserWarning that is difficult to filter that the acquisition model is correctly clipped out of the temperature range.

============================================================ warnings summary ============================================================
../../../../ska3/test/lib/python3.10/site-packages/chandra_aca/star_probs.py:390
  /proj/sot/ska3/test/lib/python3.10/site-packages/chandra_aca/star_probs.py:390: UserWarning: 
  Model grid-local-quadratic-2023-05.fits.gz computed between 5.0 <= mag <= 10.75, clipping input mag(s) outside that range.
    warnings.warn(

Interface impacts

Testing

Unit tests

• Linux

Independent check of unit tests by @taldcroft

• Mac (ran tests in this branch and saw no warnings)

Functional tests

No functional testing.

[taldcroft]

The changes looks reasonable to me.