test: getgroups() may contain duplicate GIDs

Some systems may have multiple group names with the same group ID, in
which case getgroups() returns duplicate values, where `id -G` will
filter the duplicates. Unique and sort the arrays so they can be
compared.

PR-URL: https://github.com/nodejs/node/pull/10389


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>