selftests: forwarding: Add a test case for ARP suppression

ARP suppression allows the Linux bridge to answer ARP requests on behalf
of remote hosts. It reduces the amount of packets a VTEP needs to flood.

This test verifies that ARP suppression on / off works when a neighbour
exists and when it does not exist. It does so by sending an ARP request
from a host connected to one VTEP and checking whether it was received
by a second VTEP.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>