Mark slow tests

[mkelley]

Mark the slowest tests as "slow" and only run them on CI once. Also enable parallel execution on the testing environment that runs alldeps and the slow tests.

This PR requires pytest-astropy>=0.10, which supports the use of pytest.mark.slow.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.24%. Comparing base (36285d0) to head (416ec92).
⚠️ Report is 67 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #385      +/-   ##
==========================================
- Coverage   76.84%   75.24%   -1.60%     
==========================================
  Files          78       78              
  Lines        6974     6981       +7     
==========================================
- Hits         5359     5253     -106     
- Misses       1615     1728     +113     

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