EmbeddedSpecRunner/Compiler ClassLoader support

[AndreasTu]

EmbeddedSpecRunner and EmbeddedSpecCompiler now support the construction with a ClassLoader.
This allows the use of these classes in an OSGi environment, where the class imports in the embedded spec are not visible by the Spock OSGi bundle ClassLoader

[codecov]

Codecov Report

Attention: Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.90%. Comparing base (2c7db77) to head (07202ed).
Report is 122 commits behind head on master.

Files with missing lines	Patch %	Lines
...c/main/groovy/spock/util/EmbeddedSpecRunner.groovy	77.77%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1988      +/-   ##
============================================
+ Coverage     80.44%   81.90%   +1.45%     
- Complexity     4337     4604     +267     
============================================
  Files           441      448       +7     
  Lines         13534    14417     +883     
  Branches       1707     1822     +115     
============================================
+ Hits          10888    11808     +920     
+ Misses         2008     1936      -72     
- Partials        638      673      +35     

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