add test for one method in diagnose.

[tejal29]

Add test for one of the methods in diagnose.

[codecov-io]

Codecov Report

Merging #2238 into master will increase coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2238      +/-   ##
==========================================
+ Coverage   60.03%   60.14%   +0.11%     
==========================================
  Files         185      185              
  Lines        7794     7794              
==========================================
+ Hits         4679     4688       +9     
+ Misses       2728     2719       -9     
  Partials      387      387

Impacted Files	Coverage Δ
pkg/skaffold/docker/parse.go	85.95% <100%> (ø)	⬆️
pkg/skaffold/runner/diagnose.go	13.04% <0%> (+13.04%)	⬆️

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