selftests: forwarding: Add a test case for ARP decapsulation

Verify that ARP packets are correctly decapsulated by the ingress VTEP
by removing the neighbours configured on both VLAN interfaces and
running a ping test.

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