Prepare testinstall testsuite for --bare

[fingolfin]

• Better {Primitive,Transitive}GroupsAvailable fallbacks
• Deal with undefined DefaultReportDiffColors
• Adjust some more tests to work with --bare

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.73%. Comparing base (0a43f8b) to head (4852fbb).

Files with missing lines	Patch %	Lines
lib/test.gi	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6406   +/-   ##
=======================================
  Coverage   78.73%   78.73%           
=======================================
  Files         685      685           
  Lines      293313   293314    +1     
  Branches     8672     8646   -26     
=======================================
+ Hits       230940   230943    +3     
  Misses      60552    60552           
+ Partials     1821     1819    -2     

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[ThomasBreuer]

Thanks.