Do not enforce JUnit as a platform

[alexander-yevsyukov]

This PR changes the way we declare the dependency on JUnit as a platform.

Previously, we used enforcedPlatform(). This caused the validation error during publishing of the previous PR because we played too hard with the dependency.

Now we changed it to be just platform(), so that projects using TestLib can use newer JUnit versions without waiting for the new version of TestLib, which enforces it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.67%. Comparing base (af04e7d) to head (d477142).
Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master       #8   +/-   ##
=========================================
  Coverage     60.67%   60.67%           
  Complexity      113      113           
=========================================
  Files            26       26           
  Lines           562      562           
  Branches         20       20           
=========================================
  Hits            341      341           
  Misses          213      213           
  Partials          8        8           

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