selftests: forwarding: vxlan_bridge_1d: Add flood test

Test that when sending traffic to an unlearned MAC address, the traffic
is flooded to both remote VXLAN endpoints.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>