Add more tests

[olexandr-konovalov]

Needed:

• some more basic tests in tst/testinstall/ to check that it works
• longer and very comprehensive check for teststandard and testextra (see Extended primitive groups library for degrees up to 4095. gap-system/gap#818 for some)

[olexandr-konovalov]

Note for myself: testextra/primsan.txt is a test which checks primitive groups of degree up to 999. See also some testing performed while reviewing gap-system/gap#818

[olexandr-konovalov]

Some work toward this has been done in #16 - to be continued.

[olexandr-konovalov]

Code coverage is 99%, but perhaps some sanity checks would still be useful.

[fingolfin]

Code coverage is 99% because 99.5% of the "code" in this repository are in fact data files, which are trivially covered because we read them. For the actual code, coverage right now is about 60%...