Only run a test which requires a shell on unix

[ChrisJefferson]

Fixes #3332

[olexandr-konovalov]

Like this! Note the cat error though: https://travis-ci.org/gap-system/gap/jobs/579405701

testing: /home/travis/build/gap-system/gap/tst/testinstall/testunix/streamio.t\
st
     392 ms (269 ms GC) and 123KB allocated for testunix/streamio.tst
testing: /home/travis/build/gap-system/gap/tst/testinstall/testunix/streams.ts\
t
# line 21 of 27 (77%)/bin/cat: -: Input/output error
     390 ms (272 ms GC) and 77.3KB allocated for testunix/streams.tst

[coveralls]

Coverage remained the same at 84.407% when pulling 209fa8b on ChrisJefferson:fixcygwintest into c11ced9 on gap-system:master.

[ChrisJefferson]

I think that's always been there (on and off)