Issue #1227: Fix module testing not working due to call to JUnit internal API

[Choosechee]

I've replaced a use of the internal JUnit class org.junit.platform.commons.util.ClassUtils with a new custom org.spockframework.util.ClassUtilclass. This fixes issue #1227 (tested with my project).

[AndreasTu]

@Choosechee Thank you very much for your contribution.

[AndreasTu]

@leonard84 I have fixed the review finding for the PR. Could you have another look please?
It would be great, if this would land in 2.4, because it also breaks the OSGi usage with JUnit 6 do the the same issue as the modules.

[AndreasTu]

This PR will also fix #2231

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.14%. Comparing base (b8f0f84) to head (2ea89b1).

Files with missing lines	Patch %	Lines
...c/main/java/org/spockframework/util/ClassUtil.java	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #2187   +/-   ##
=========================================
  Coverage     82.13%   82.14%           
- Complexity     4789     4794    +5     
=========================================
  Files           468      469    +1     
  Lines         14952    14961    +9     
  Branches       1888     1888           
=========================================
+ Hits          12281    12289    +8     
- Misses         1983     1984    +1     
  Partials        688      688           

Files with missing lines	Coverage Δ
...ain/java/org/spockframework/runtime/SpockNode.java	80.85% <100.00%> (ø)
...c/main/java/org/spockframework/util/ClassUtil.java	88.88% <88.88%> (ø)

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[leonard84]

Thanks @Choosechee