selftests: net: bridge: add vlan mcast_membership_interval test

Add a test which changes the new per-vlan mcast_membership_interval and
verifies that a newly learned mdb entry would expire in that interval.

TEST: Vlan mcast_membership_interval global option default value    [ OK ]
TEST: Vlan 10 mcast_membership_interval option changed to 200       [ OK ]
TEST: Vlan 10 mcast_membership_interval mdb entry expire            [ OK ]

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>