Fix ExtensionException in OSGi environment for global extension

[AndreasTu]

Implement the same resource logic with ContextClassLoader as JUnit platform.

Fixes #2076

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.87%. Comparing base (9d1e819) to head (b6e4794).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #2079   +/-   ##
=========================================
  Coverage     81.87%   81.87%           
+ Complexity     4687     4686    -1     
=========================================
  Files           452      452           
  Lines         14658    14656    -2     
  Branches       1845     1845           
=========================================
- Hits          12001    12000    -1     
  Misses         1976     1976           
+ Partials        681      680    -1     

Files with missing lines	Coverage Δ
...spockframework/runtime/ExtensionClassesLoader.java	68.42% <100.00%> (ø)
...n/java/org/spockframework/util/ReflectionUtil.java	89.82% <100.00%> (+0.47%)	⬆️