Skip tests action if only other actions changed

[michaelosthege]

The tests don't need to run if it's just other workflows that are being changed.

What is this PR about?
Reducing the number of unnecessary tests.yml workflows.

This is mostly in preparation of working on #5974, which will trigger a ton of CI.

The change needs to be made beforehand (I tried to do it in #5975), because the diff is evaluated between the branches, not just the commits.

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues (preferably in nice commit messages)
• Are the changes covered by tests and docstrings?
• Fill out the short summary sections 👇

Major / Breaking Changes

None

Bugfixes / New features

None

Docs / Maintenance

Updated trigger conditions for the tests GH action to reduce the number of unnecessary trigger events.

[codecov]

Codecov Report

Merging #5976 (a94b681) into main (2583b7f) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #5976      +/-   ##
==========================================
+ Coverage   89.35%   89.38%   +0.02%     
==========================================
  Files          73       73              
  Lines       13251    13251              
==========================================
+ Hits        11841    11844       +3     
+ Misses       1410     1407       -3     

Impacted Files	Coverage Δ
pymc/parallel_sampling.py	86.79% <0.00%> (+0.99%)	⬆️