Make GAP_pkg_FOOBAR/test/runttests.jl files do something useful

[fingolfin]

Naively we could simply invoke GAP.Globals.TestPackage("FOOBAR"). However, in the current model that exits GAP (and hence OSCAR / Julia), which we don't want...

In the short term, we could hack TestDirectory to handle exitGAP := true differently... But most GAP packages call FORCE_QUIT_GAP(1); after their TestDirectory call...

Short term solution: temporarily replace the GAP functions QuitGap (called by TestDirectory) and FORCE_QUIT_GAP with our own, which just record the exit code set by GAP, i.e., GapExitCode(), to determine the test outcome

We could add a helper function doing all that to GAP.jl. It ain't pretty, but it should work for the majority of packages

Long term:

• get GAP packages to drop those FORCE_QUIT_GAP calls
• add a dedicated (and documented) interface in GAP to indicate the test result
• modify TestDirectory to use that, and/or add a sanctioned way to override the exitGAP option... and/or add a better named alternative to exitGAP (however, most GAP packages will want to stay compatible with older GAP versions, switching to such a new option would be a hassle for them)