Add test coverage for checks

[mszostok]

Description

Changes proposed in this pull request:

• Add unit test coverage for checks

Related issue(s)

• Add test coverage to internal pkg #14

[codecov]

Codecov Report

Merging #48 into master will increase coverage by 14.33%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master      #48       +/-   ##
===========================================
+ Coverage   29.47%   43.81%   +14.33%     
===========================================
  Files          11       12        +1     
  Lines         346      372       +26     
===========================================
+ Hits          102      163       +61     
+ Misses        239      207       -32     
+ Partials        5        2        -3     

Impacted Files	Coverage Δ
internal/check/api.go	53.12% <100.00%> (+9.79%)	⬆️
internal/printer/tty.go	100.00% <100.00%> (ø)
internal/check/valid_owner.go	9.90% <0.00%> (+9.90%)	⬆️
internal/check/not_owned_file.go	11.70% <0.00%> (+11.70%)	⬆️
internal/check/valid_syntax.go	100.00% <0.00%> (+12.50%)	⬆️
internal/check/duplicated_pattern.go	100.00% <0.00%> (+21.42%)	⬆️
internal/check/file_exists.go	100.00% <0.00%> (+22.72%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 89d9a5f...0bf22dc. Read the comment docs.