TST: Silence specutils warning in fixture

[pllim]

Description

This pull request is to handle new warning that pops up due to astropy/specutils#922 in newly released specutils.

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 change log needed? If yes, is it added to CHANGES.rst?
• Is a milestone set? Milestone is only currently required for PRs related to Imviz MVP.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.89%. Comparing base (c523362) to head (6871b1f).
Report is 2570 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1093   +/-   ##
=======================================
  Coverage   75.89%   75.89%           
=======================================
  Files          82       82           
  Lines        6278     6278           
=======================================
  Hits         4765     4765           
  Misses       1513     1513           

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[kecnry]

This will be needed with specutils 1.7 which is now pinned in #1060