Additional unit tests for new code

[craigthomas]

This PR adds additional unit tests for lines of code that were missing coverage. This PR closes #42

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.72%. Comparing base (b802ca0) to head (904cea4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
+ Coverage   92.87%   94.72%   +1.85%     
==========================================
  Files           4        4              
  Lines         660      664       +4     
==========================================
+ Hits          613      629      +16     
+ Misses         47       35      -12     

Files	Coverage Δ
chip8/cpu.py	98.96% <100.00%> (+2.51%)	⬆️

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