selftests: forwarding: vxlan_bridge_1d: Add a TTL test

This tests whether TTL of VXLAN envelope packets is properly set based
on the device configuration.

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>