zdtm: set additional groups along with uids and gids

Currently zdtm doesn't set additional groups when running
tests. It sets uids and gids to some non-root values, but
additional groups remain untouched, which allows tests
to inherit groups list, which leads to different
behaviour depending on a way you run tests.

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>