Add local fixtures in test_polydata

[adeak]

The test_polydata.py module had a handful of global meshes that were being copied all over the place. This PR replaces them with module-level fixtures.

One non-trivial change is that the now-local sphere fixture (coming from pv.Sphere) shadows the global sphere fixture (coming from pv.examples.load_sphere, ultimately reading a file with data arrays). This doesn't seem like an issue and the tests run fine.

[adeak]

GitHub actions are still flaky. We can wait until they come back; this one needs solid CI.

[codecov]

Codecov Report

Merging #1326 (18e2b1d) into master (d6ada7b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1326   +/-   ##
=======================================
  Coverage   90.05%   90.05%           
=======================================
  Files          56       56           
  Lines       12181    12181           
=======================================
  Hits        10969    10969           
  Misses       1212     1212           

[akaszynski]

I restarted CI and it seems that it ran fine. Test run locally with pytest -vx -n 4 on Linux.

I see no issue using a different sphere fixture other than we might consider replacing the global fixture in a later PR with one like this (if we're fine giving up the data_arrays).

[akaszynski]

You've got merge permission and can fire when ready.

[adeak]

Thanks!