selftests: forwarding: vxlan_bridge_1d: Add unicast test

Test that when sending traffic to a learned MAC address, the traffic is
forwarded accurately only to the right endpoint.

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>