netdevsim: add selftest for forwarding skb between connected ports

Connect two netdevsim ports in different namespaces together, then send
packets between them using socat.

Signed-off-by: David Wei <dw@davidwei.uk>
Reviewed-by: Maciek Machnikowski <maciek@machnikowski.net>
Signed-off-by: David S. Miller <davem@davemloft.net>