Set fork as the starting method of a sub-process

[GuyAv46]

• Explicitly set the sub-process start method to 'fork', as the default method is different among different OSs, and we rely on forking the memory (local functions, global Defaults struct, and more)
• Setting some default values in _runTest to be no-op callable so we can always call them (and not have to check if they're None)

To properly run with parallelism on macOS, you'll have to call RLTest with OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES

[codecov-commenter]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (d291d4c) 32.50% compared to head (bad25f6) 32.52%.

Files	Patch %	Lines
RLTest/__main__.py	0.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
+ Coverage   32.50%   32.52%   +0.01%     
==========================================
  Files          17       17              
  Lines        2587     2586       -1     
==========================================
  Hits          841      841              
+ Misses       1746     1745       -1     

Flag	Coverage Δ
unittests	32.52% <0.00%> (+0.01%)	⬆️

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