test: avoid FP in chroot-credentials.sh for different group list order

test: avoid FP in chroot-credentials.sh for different group list order

On my openSUSE:Tumbleweed system, I get a false positive test failure
in the above 'check-root' test because the group lists inside and
outside the chroot have a different order:

  ++ chroot --userspec=berny / id -G
  ++ id -G berny
  + test '100 454 457 480 492' = '100 480 492 457 454'
  + fail=1

* tests/misc/chroot-credentials.sh (num_sort): Add function to sort
group lists, and use it in the test cases which test multiple groups.