Add unit-tests coverage for dup_pattern checker

[mszostok]

Description

Changes proposed in this pull request:

• Add test coverage for internal pkg

Related issue(s)

Partially solves #14

[codecov]

Codecov Report

Merging #43 into master will decrease coverage by 64.65%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master      #43       +/-   ##
===========================================
- Coverage   82.92%   18.26%   -64.66%     
===========================================
  Files           2       10        +8     
  Lines          41      312      +271     
===========================================
+ Hits           34       57       +23     
- Misses          6      252      +246     
- Partials        1        3        +2     

Impacted Files	Coverage Δ
internal/check/duplicated_pattern.go	78.57% <100.00%> (ø)
pkg/codeowners/owners.go	81.08% <100.00%> (ø)
internal/context/check.go	100.00% <0.00%> (ø)
internal/check/not_owned_file.go	0.00% <0.00%> (ø)
internal/check/valid_owner.go	0.00% <0.00%> (ø)
internal/check/api.go	17.24% <0.00%> (ø)
internal/envconfig/envconfig.go	100.00% <0.00%> (ø)
internal/check/valid_owner_error.go	0.00% <0.00%> (ø)
internal/check/file_exists.go	0.00% <0.00%> (ø)

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