selftests: forwarding: bridge_mdb_max: Add a new selftest

Add a suite covering mcast_n_groups and mcast_max_groups bridge features.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>