TEST: Style cleanups, simpler fixtures

[effigies]

Hopefully I'm not overlapping with #876, as it looks like @djarecka and I were both going through my backlog of suggestions on #865.

Also fixes the style issue that's going to be making everybody's Travis fail... Sorry.

[codecov]

Codecov Report

Merging #878 into pytest will increase coverage by 0.77%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           pytest     #878      +/-   ##
==========================================
+ Coverage   82.76%   83.53%   +0.77%     
==========================================
  Files          97       97              
  Lines       12404    12403       -1     
  Branches     2185     2185              
==========================================
+ Hits        10266    10361      +95     
+ Misses       1750     1678      -72     
+ Partials      388      364      -24

Impacted Files	Coverage Δ
nibabel/optpkg.py	97.05% <ø> (-0.09%)	⬇️
nibabel/streamlines/tractogram_file.py	100% <0%> (+5%)	⬆️
nibabel/streamlines/trk.py	94.62% <0%> (+16.45%)	⬆️
nibabel/streamlines/tck.py	99.46% <0%> (+19.78%)	⬆️

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