Improve test coveragev

[mdboom]

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[codecov-io]

Codecov Report

Merging #6 into master will increase coverage by 2.96%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
+ Coverage   89.82%   92.78%   +2.96%     
==========================================
  Files          14       14              
  Lines        3085     3062      -23     
==========================================
+ Hits         2771     2841      +70     
+ Misses        314      221      -93

Impacted Files	Coverage Δ
pytoshop/util.py	94.69% <ø> (+11.61%)	⬆️
pytoshop/image_resources.py	90.03% <ø> (+2.66%)	⬆️
pytoshop/layers.py	94.55% <ø> (+4.67%)	⬆️
pytoshop/image_data.py	96.15% <100%> (+16.15%)	⬆️
pytoshop/blending_range.py	96.77% <0%> (+12.9%)	⬆️

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