selftests: net: bridge: add vlan mcast_router tests

Add tests for the new per-port/vlan mcast_router option, verify that
unknown multicast packets are flooded only to router ports.

TEST: Port vlan 10 option mcast_router default value                [ OK ]
TEST: Port vlan 10 mcast_router option changed to 2                 [ OK ]
TEST: Flood unknown vlan multicast packets to router port only      [ OK ]

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