Exclude test files from coverage

[brettcannon]

No description provided.

[codecov]

Codecov Report

Merging #11 into master will increase coverage by 8.98%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master       #11      +/-   ##
===========================================
+ Coverage   91.01%   100.00%   +8.98%     
===========================================
  Files           5         2       -3     
  Lines         512       297     -215     
===========================================
- Hits          466       297     -169     
+ Misses         46         0      -46     

Impacted Files	Coverage Δ
src/cli.rs	100.00% <ø> (+1.44%)	⬆️
src/lib.rs	100.00% <100.00%> (+16.11%)	⬆️
tests/lib_system_tests.rs

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