selftests: forwarding: vxlan_bridge_1d: Add an ECN encap test

Test that ECN bits in the VXLAN envelope are correctly deduced from the
overlay packet.

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>