Add fail_fast option and added test

[coreman14]

No description provided.

[whtsky]

I’m trying to understand why tests are taking so long.. https://github.com/whtsky/pixelmatch-py/runs/5528003953?check_suite_focus=true

It feels like a bug in pytest to me

[codecov]

Codecov Report

Merging #144 (69e3ac1) into master (fe21125) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head 69e3ac1 differs from pull request most recent head c0afd40. Consider uploading reports for the commit c0afd40 to get more accurate results

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
+ Coverage   97.93%   97.95%   +0.02%     
==========================================
  Files           5        5              
  Lines         145      147       +2     
==========================================
+ Hits          142      144       +2     
  Misses          3        3              

Impacted Files	Coverage Δ
pixelmatch/contrib/PIL.py	100.00% <ø> (ø)
pixelmatch/core.py	91.66% <100.00%> (+0.49%)	⬆️

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