Set O_TRUNC on mock output files

[LandonTClipp]

This fixes #604

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (5cf2282) 74.29775% compared to head (53b9d39) 74.29775%.

❗ Current head 53b9d39 differs from pull request most recent head 6899a71. Consider uploading reports for the commit 6899a71 to get more accurate results

Additional details and impacted files

@@              Coverage Diff              @@
##              master        #606   +/-   ##
=============================================
  Coverage   74.29775%   74.29775%           
=============================================
  Files              7           7           
  Lines           2136        2136           
=============================================
  Hits            1587        1587           
  Misses           421         421           
  Partials         128         128           

Impacted Files	Coverage Δ
pkg/outputter.go	41.20603% <0.00000%> (ø)

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