TEST: Refactor NetCDF tests to be more pytest friendly

[effigies]

No description provided.

[codecov]

Codecov Report

Merging #879 into pytest will decrease coverage by 0.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           pytest     #879      +/-   ##
==========================================
- Coverage   89.75%   89.51%   -0.24%     
==========================================
  Files          97       97              
  Lines       12403    12156     -247     
  Branches     2185     2185              
==========================================
- Hits        11132    10882     -250     
  Misses        927      927              
- Partials      344      347       +3

Impacted Files	Coverage Δ
nibabel/dft.py	80.98% <0%> (ø)	⬆️
nibabel/freesurfer/io.py	94.22% <0%> (ø)	⬆️
nibabel/environment.py	95% <0%> (ø)	⬆️
nibabel/casting.py	86.14% <0%> (ø)	⬆️
nibabel/openers.py	94.89% <0%> (-0.16%)	⬇️
nibabel/nicom/dicomwrappers.py	90.35% <0%> (-0.56%)	⬇️
nibabel/nicom/csareader.py	87.4% <0%> (ø)	⬆️
nibabel/ecat.py	87.7% <0%> (-0.37%)	⬇️
nibabel/volumeutils.py	82.84% <0%> (-1.3%)	⬇️
nibabel/cifti2/cifti2_axes.py	98.26% <0%> (-0.09%)	⬇️
... and 4 more

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