selftests: forwarding: vxlan_bridge_1d: Add a TOS test

Test that TOS is inherited from the tunneled packet into the envelope as
configured at the VXLAN device.

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>