Tweak tests a bit

[fingolfin]

While working on removing GAPstones (see PR #1072), I made some tweaks and improvements to various tests, mainly to speed them up. I figured we might want to merge this workindependently of the GAPstones removal. In particular, it should be uncontroversial, which the removal of GAPstones might not be.

[codecov-io]

Current coverage is 54.32% (diff: 100%)

Merging #1100 into master will decrease coverage by 0.21%

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  Files           432        432          
  Lines        224765     224765          
  Methods        3431       3431          
  Messages          0          0          
  Branches          0          0          
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- Hits         122572     122095   -477   
- Misses       102193     102670   +477   
  Partials          0          0          

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[fingolfin]

On #1072, @alex-konovalov mentioned that the new testextra dir could be hooked up to be run by tst/teststandard.g. Sure, that's easy to do.

As a matter of fact, I think we should rename the subdirs testinstall, teststandard to, say, testquick and testslow or so -- to emphasize that there is not a 1-to-1 correspondence between them, and the "testsuites". Already now, teststandard.g also runs the tests from testinstall. And testtravis runs a mishmash, too.

[markuspf]

I shuld probably mention #790, which was a (failed) attempt to do what you propose.

[fingolfin]

@markuspf Thanks for that pointer! Also https://github.com/gap-system/gap/wiki/Testing-methods-and-targets. I'll see what I can do.

[ChrisJefferson]

This is all totally fine. Will merge later unless anyone raises issues.

There are obvious followups to come -- but this can be merged as is for now.

[olexandr-konovalov]

On #1072, @alex-konovalov mentioned that the new testextra dir could be hooked up to be run by tst/teststandard.g. Sure, that's easy to do.

But has not been done. I will do this now. I will make a direct commit to master branch since this is not covered by Travis CI anyway.

[olexandr-konovalov]

Done in 6c97733