ci(tests): Add parallel_show_output=True to test environments in tox.ini

[PProfizi]

This means that test steps will always show the list of tests which passed in parallel Tox environments, even if all are successful.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.93%. Comparing base (eb4627e) to head (b1ceaa7).
Report is 3 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2328      +/-   ##
==========================================
- Coverage   83.97%   83.93%   -0.05%     
==========================================
  Files          90       90              
  Lines       10399    10399              
==========================================
- Hits         8733     8728       -5     
- Misses       1666     1671       +5     

[PProfizi]

Expected effect confirmed: https://github.com/ansys/pydpf-core/actions/runs/15418096278/job/43385649376?pr=2328#step:22:38

[rafacanton]

Thanks a lot!