Add tests for YamlConfig

[schalkms]

subsequent task of #2417

[codecov-io]

Codecov Report

Merging #2426 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #2426      +/-   ##
============================================
+ Coverage     82.87%   82.88%   +0.01%     
  Complexity     2171     2171              
============================================
  Files           356      356              
  Lines          6142     6142              
  Branches       1122     1122              
============================================
+ Hits           5090     5091       +1     
  Misses          478      478              
+ Partials        574      573       -1

Impacted Files	Coverage Δ	Complexity Δ
...itlab/arturbosch/detekt/api/internal/YamlConfig.kt	80% <0%> (+3.33%)	9% <0%> (ø)	⬇️

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