TST: Handle second warning in test_all_nan_uncert_subset

[pllim]

Description

This pull request is to handle second warning in test_all_nan_uncert_subset because pytest 8 will turn it into exception but it is hidden in older pytest. Noticed this in #2654 when pytest 8.0rc1 doesn't crash for other reasons (log: https://github.com/spacetelescope/jdaviz/actions/runs/7450389588/job/20269179809?pr=2654).

You can prove this to yourselves by pip install pytest --pre -U locally and then run the test suite.

xref #2643

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (03edac6) 91.53% compared to head (4e218ac) 91.53%.

Files	Patch %	Lines
...default/plugins/model_fitting/tests/test_plugin.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2656      +/-   ##
==========================================
- Coverage   91.53%   91.53%   -0.01%     
==========================================
  Files         161      161              
  Lines       20056    20062       +6     
==========================================
+ Hits        18358    18363       +5     
- Misses       1698     1699       +1     

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[rosteen]

Test passes locally with 8.0.0rc1, thanks.