selftests: forwarding: vxlan_bridge_1d: Add an ECN decap test

Test that when decapsulating from VXLAN, the values of inner and outer
TOS are handled appropriately. Because VXLAN driver on its own won't
produce the arbitrary TOS combinations necessary to test this feature,
simply open-code a single ICMP packet and have mausezahn assemble it.

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>