Change spek and assertj to compile only dependencies for detekt-test consumers

[arturbosch]

Closes #3082

Tested with the intellij plugin.
No need to exclude these dependencies anymore:

   testImplementation("io.gitlab.arturbosch.detekt:detekt-test-utils:$detektVersion") {
       exclude(group = "org.assertj")
       exclude(group = "org.spekframework.spek2")
   }

[codecov]

Codecov Report

Merging #3152 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #3152   +/-   ##
=========================================
  Coverage     79.52%   79.52%           
  Complexity     2592     2592           
=========================================
  Files           437      437           
  Lines          7852     7852           
  Branches       1495     1495           
=========================================
  Hits           6244     6244           
  Misses          817      817           
  Partials        791      791           

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