gplazma: authzdb add test case

the authDb plugin supports mapping username principals to multiple gid
principals. But there are no test that verify that functionality.

This patch adds such a test.

Ticket: 7681
Acked-by: Paul
Target: trunk
Require-book: no
Require-notes: no
scripts: allow java8 runtime

Acked-by: Paul Millar
Target: master, 2.9, 2.8, 2.7, 2.6
Require-book: no
Require-notes: no