Fix conftest import

[melanieclarke]

Closes #9714

Reorganize test data to avoid importing directly from conftest files.

The biggest change here is to the extract_1d conftest file. Currently, only a couple of the fixtures there have been converted to functions to allow them to be reused elsewhere. Since many of them are useful synthetic structures for testing a wide variety of spectral modes, I went ahead and converted almost all the fixtures to functions and moved their implementation to the helpers module. I left a few as fixture-only input data, for tests specific to extract_1d utilities. They could be moved to the helpers later, if they turn out to be useful elsewhere.

Tasks

• If you have a specific reviewer in mind, tag them.
• add a build milestone, i.e. Build 12.0 (use the latest build if not sure)
• Does this PR change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see changelog readme for instructions)
  • update or add relevant tests
  • update relevant docstrings and / or docs/ page
  • start a regression test and include a link to the running job (click here for instructions)
    • Do truth files need to be updated ("okified")?
      • after the reviewer has approved these changes, run okify_regtests to update the truth files
• if a JIRA ticket exists, make sure it is resolved properly

[melanieclarke]

Regtests shouldn't be needed, but running them anyway just in case:
https://github.com/spacetelescope/RegressionTests/actions/runs/16652526399

All passing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.11%. Comparing base (3c133e8) to head (35d4022).
⚠️ Report is 268 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9715   +/-   ##
=======================================
  Coverage   82.11%   82.11%           
=======================================
  Files         367      368    +1     
  Lines       37482    37484    +2     
=======================================
+ Hits        30778    30780    +2     
  Misses       6704     6704           

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[pllim]

LGTM in general, just some minor comments. Thanks!

I wonder if we should add __all__ to test helper modules...

[melanieclarke]

I wonder if we should add __all__ to test helper modules...

Probably. I'll add it.

[pllim]

Thanks!

[emolter]

Thanks Melanie, looks great overall. I had a question about some docstring changes