TST: promote warnings to errors

[neutrinoceros]

There are several warnings popping up in test sessions, some of which indicating future behavior change in dependencies that could be handled. Turning them into hard failures in CI is a first step towards future proofing and ensures that no new warnings are introduced silently.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 23.69%. Comparing base (3373f93) to head (3b28b2b).
⚠️ Report is 25 commits behind head on master.

Files with missing lines	Patch %	Lines
vip_hci/fm/negfc_simplex.py	0.00%	1 Missing ⚠️
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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #678       +/-   ##
===========================================
- Coverage   62.28%   23.69%   -38.59%     
===========================================
  Files          11       87       +76     
  Lines         928    16835    +15907     
===========================================
+ Hits          578     3989     +3411     
- Misses        350    12846    +12496     

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