Add test for invalid -skip-regexp

[maratori]

No description provided.

[codecov]

Codecov Report

Merging #10 into master will increase coverage by 11.76%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #10       +/-   ##
=========================================
+ Coverage   88.23%   100%   +11.76%     
=========================================
  Files           1      1               
  Lines          17     17               
=========================================
+ Hits           15     17        +2     
+ Misses          1      0        -1     
+ Partials        1      0        -1

Impacted Files	Coverage Δ
pkg/testpackage/testpackage.go	100% <0%> (+11.76%)	⬆️

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