selftests: forwarding: vxlan_bridge_1d: Add ping test

Test end-to-end reachability between local and remote endpoints.

Note that because learning is disabled on the VXLAN device, the ICMP
requests will end up being flooded to all remotes.

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>