Allow for specification of 'var_names' in 'mock_sample'

[tomicapretto]

This PR adds var_names to the call to pm.sample_prior_predictive in mock_sample.
I'm looking forward to speed up tests in Bambi, which uses var_names under the hood.

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📚 Documentation preview 📚: https://pymc--7906.org.readthedocs.build/en/7906/

[williambdean]

Test failures seem unrelated...
But additions look good

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.62%. Comparing base (9f653a6) to head (7a3db78).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7906      +/-   ##
==========================================
- Coverage   92.94%   91.62%   -1.33%     
==========================================
  Files         116      116              
  Lines       18851    18852       +1     
==========================================
- Hits        17522    17274     -248     
- Misses       1329     1578     +249     

Files with missing lines	Coverage Δ
pymc/testing.py	90.00% <100.00%> (+0.02%)	⬆️

... and 4 files with indirect coverage changes

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[tomicapretto]

Thanks @williambdean @jessegrabowski