Speed up regtests by reorganizing test_nirspec_ifu_spec3

[braingram]

This is a minor reorganization of tests in test_nirspec_ifu_spec3 to more efficiently run them in parallel.

This reduces regtests runtime by about 8 minutes.

jwst/regtest/test_nirspec_ifu_spec3.py contains 2 of the slower regtests since the regtests run in parallel scoped by module these 2 tests end up in the same scope given to 1 worker. This worker then spends over 2 hours on those 2 tests (run serially). This PR splits jwst/regtest/test_nirspec_ifu_spec3.py putting the 2 slow fixtures in different files/modules so they can be run on different workers.

Ideally we'd start the slowest tests first so they could toil away while other workers run other tests. If we instead start them later in the test run our overall run will likely end up taking as we wait for the slow test to finish. However this is currently not possible with pytest-xdist (see pytest-dev/pytest-xdist#778).

With this PR regtests ran in 1hr 8 minutes: https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/1745/ (with only unrelated failures).

Tasks

• request a review from someone specific, to avoid making the maintainers review every PR
• add a build milestone, i.e. Build 11.3 (use the latest build if not sure)
• Does this PR change user-facing code / API?
  • add an entry to CHANGES.rst within the relevant release section (otherwise add the no-changelog-entry-needed label to this PR)
  • 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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.86%. Comparing base (eab0532) to head (e860360).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8811   +/-   ##
=======================================
  Coverage   61.86%   61.86%           
=======================================
  Files         377      377           
  Lines       38911    38911           
=======================================
  Hits        24071    24071           
  Misses      14840    14840           

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[melanieclarke]

Looks good to me! I've asked @jemorrison to take a quick look too, since she added the pixel replace test.

[jemorrison]

Looks ok to me