Add pytest-xdist to tox and run multi-threaded

[JSCU-CNI]

This PR significantly improves the total time the tests take to complete on most systems. Limited to four threads to keep CI/CD workers happy.

Before on x86_64:

$ tox -e py313
...
2028 passed, 1 skipped in 120.77s (0:02:00)

After on x86_64:

$ tox -e py313
...
2028 passed, 1 skipped, 1 warning in 53.27s

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.27%. Comparing base (8ba80d6) to head (89946d2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1089      +/-   ##
==========================================
+ Coverage   79.23%   79.27%   +0.03%     
==========================================
  Files         345      345              
  Lines       30656    30656              
==========================================
+ Hits        24290    24302      +12     
+ Misses       6366     6354      -12     

Flag	Coverage Δ
unittests	79.27% <ø> (+0.03%)	⬆️

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[Schamper]

Doesn't seem all that much faster on GitHub but it's a nice local improvement 😂 I'd actually been running this locally for a bit already.