selftest: Show that 'allow trusted domains = no' firewalls Unix User|Group

selftest: Show that 'allow trusted domains = no' firewalls Unix User|Group

UNEXPECTED(failure): samba3.blackbox.smbclient_auth.plain.local_creds.smbclient //LOCALSHARE4/forceuser_unixonly as user(simpleserver)
REASON: Exception: Exception: tree connect failed: NT_STATUS_AUTHENTICATION_FIREWALL_FAILED

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15469

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
(cherry picked from commit ad0c0dd071401d98f0b7f595efbdf5312a165ab4)