Add a few extra tests for improved coverage

[rhpvorderman]

Checklist

• Pull request details were added to CHANGELOG.rst

[codecov]

Codecov Report

Merging #47 (13daae3) into develop (9ea5911) will increase coverage by 2.64%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop      #47      +/-   ##
===========================================
+ Coverage    96.68%   99.33%   +2.64%     
===========================================
  Files            2        2              
  Lines          151      151              
===========================================
+ Hits           146      150       +4     
+ Misses           5        1       -4     

Impacted Files	Coverage Δ
src/isal/igzip.py	99.27% <0.00%> (+2.91%)	⬆️

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