Add junit6 to spock-testkit

[AndreasTu]

This shall ensure the execution with JUnit 6 is possible.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.07%. Comparing base (a29c310) to head (e64e180).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #2236   +/-   ##
=========================================
  Coverage     82.07%   82.07%           
  Complexity     4756     4756           
=========================================
  Files           465      465           
  Lines         14872    14872           
  Branches       1877     1877           
=========================================
  Hits          12206    12206           
  Misses         1978     1978           
  Partials        688      688           

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[Vampire]

Wouldn't it instead make sense to have a second test task in spock-specs that runs with JUnit 6 so that all tests are executed? There could also be problems in specific paths, like when using data driven tests and so on.

[AndreasTu]

We could do that, but I did not want to double the test-execution time.
I would be more specific, if there is a regression add a test for JUnit6 there instead of executing everything twice.

[Vampire]

@leonard84 what's your take on this? :-)

[leonard84]

If we want to test the SpockEngine compatibility, then I'd reuse spock-testkit.

[AndreasTu]

@leonard84 done