Add test coverage for checks

[mszostok]

Description

Changes proposed in this pull request:

• Add unit test coverage for checks

Related issue(s)

• https://github.com/mszostok/codeowners-validator/issues/14

[codecov[bot]]

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%)	:arrow_up:
internal/printer/tty.go	100.00% <100.00%> (ø)
internal/check/valid_owner.go	9.90% <0.00%> (+9.90%)	:arrow_up:
internal/check/not_owned_file.go	11.70% <0.00%> (+11.70%)	:arrow_up:
internal/check/valid_syntax.go	100.00% <0.00%> (+12.50%)	:arrow_up:
internal/check/duplicated_pattern.go	100.00% <0.00%> (+21.42%)	:arrow_up:
internal/check/file_exists.go	100.00% <0.00%> (+22.72%)	:arrow_up:

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