Add tests for reading legend-testdata histograms

[apmypb]

Resolves #44

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.56%. Comparing base (c020691) to head (5a78148).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #51      +/-   ##
==========================================
+ Coverage   56.43%   56.56%   +0.13%     
==========================================
  Files           7        7              
  Lines         769      769              
==========================================
+ Hits          434      435       +1     
+ Misses        335      334       -1     

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