Convert tests/unit/mesh to PyTest.

[trexfeathers]

test_MeshXY was a mess (my fault originally. I've done my best to find a pattern that limits the changes, to help with reviewing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.00%. Comparing base (fee1d96) to head (d07705c).
⚠️ Report is 111 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6631      +/-   ##
==========================================
+ Coverage   89.95%   90.00%   +0.04%     
==========================================
  Files          91       91              
  Lines       24447    24447              
  Branches     4567     4567              
==========================================
+ Hits        21992    22004      +12     
+ Misses       1686     1676      -10     
+ Partials      769      767       -2     

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[ESadek-MO]

Please could you change the unittest.mock calls in test_MeshCoord?

Dir as a whole looks good, a couple of changes. There are one or two comments which are just comments, not change requests, feel free to resolve those once you've read them.

There're some massive files, which possibly haven't had great attention to detail from me, but I think it's 80 to 20.

[trexfeathers]

Thanks @ESadek-MO, looks like a dropped the ball quite a bit there.

[ESadek-MO]

One more. Feel free to veto.

[ESadek-MO]

Thanks @trexfeathers