Add unit tests

[matewolf]

Related to #11 .

Pull request contains unit tests for all the component that is not just a wrapper of a third party package. I made major changes in code to reach testability: I created mocks and new interfaces and refactored the business logic.

I also found some issues during testing. The affected tests are commented. New issues will be created soon based on findings.

[codecov]

Codecov Report

Merging #176 (d1288c8) into main (508d234) will increase coverage by 39.96%.
The diff coverage is 29.18%.

@@            Coverage Diff             @@
##            main     #176       +/-   ##
==========================================
+ Coverage   0.00%   39.96%   +39.96%     
==========================================
  Files          1        6        +5     
  Lines         67      663      +596     
==========================================
+ Hits           0      265      +265     
- Misses        67      389      +322     
- Partials       0        9        +9     

Files Changed	Coverage Δ
internal/config/mock.go	0.00% <0.00%> (ø)
internal/utils/comment/mock.go	0.00% <0.00%> (ø)
internal/utils/issue/mock.go	0.00% <0.00%> (ø)
internal/config/config.go	33.00% <38.59%> (ø)
internal/utils/comment/comment.go	44.44% <45.45%> (ø)
internal/utils/issue/issue.go	59.17% <50.00%> (ø)

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[justaugustus]

branch conflict needs to be resolved