Refactor test case virtual dataset creation into pytest fixture.

[norlandrhagen]

Moved logic from test_combine_by_coords for creating a virtual dataset into a fixture for re-use.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.33%. Comparing base (2c83531) to head (c33fe2f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
+ Coverage   90.29%   90.33%   +0.04%     
==========================================
  Files          14       14              
  Lines         938      942       +4     
==========================================
+ Hits          847      851       +4     
  Misses         91       91              

Flag	Coverage Δ
unittests	90.33% <100.00%> (+0.04%)	⬆️

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[TomNicholas]

I'm not sure this change makes sense... I can see why it might be nice to factor out the creation of the virtual datasets into a fixture, but the xr.combine_by_coords call should definitely still be in the test itself, because if an error is thrown it will likely be from inside that function.

EDIT: Also the xr.combine_by_coords would be present in user code, and we should test what the user would write.

[norlandrhagen]

Makes sense! Getting a bit too carried away in fixture land 😅