TST: Turning warnings into exceptions

[pllim]

To debug something Robert told me about but also just good to do in general. Also, we don't use Jenkins anymore.

RTs:

• stable: https://github.com/spacetelescope/RegressionTests/actions/runs/26292213287
• dev: https://github.com/spacetelescope/RegressionTests/actions/runs/26292184640

Fix #38

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #269   +/-   ##
=======================================
  Coverage        ?   79.09%           
=======================================
  Files           ?       34           
  Lines           ?     1612           
  Branches        ?        0           
=======================================
  Hits            ?     1275           
  Misses          ?      337           
  Partials        ?        0           

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[pllim]

OK @stscirij the log is here: https://github.com/spacetelescope/RegressionTests/actions/runs/26292184640/job/77395063193

From what I can understand there, the violation is in the "truth" files for those 2 tests. So to debug, you can go into Artifactory, grab those 2 offending truth files, open it with astropy dev or 8.0.0b1, and you should see the same warning. Then hopefully the warning message would give you enough hints to fix the problem. Once fixed, you replace the truth files, and the problem should go away. Then you can ping me here and I can rerun the RT again with this branch. Hope this helps.