Fix Boolean is getter methods not properly mocked in Groovy <= 3

[AndreasTu]

Added tests to verify that boolean is getter work according to Groovy rules.

Fixes #2131

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.04%. Comparing base (c035dd4) to head (5c55ff6).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
.../org/spockframework/runtime/GroovyRuntimeUtil.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2244      +/-   ##
============================================
+ Coverage     82.00%   82.04%   +0.04%     
  Complexity     4752     4752              
============================================
  Files           465      465              
  Lines         14871    14871              
  Branches       1878     1878              
============================================
+ Hits          12195    12201       +6     
+ Misses         1988     1982       -6     
  Partials        688      688              

Files with missing lines	Coverage Δ
.../org/spockframework/runtime/GroovyRuntimeUtil.java	82.97% <0.00%> (+6.38%)	⬆️

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[Vampire]

LGTM, thx