selftests/net: expand gro with two machine test

The test is currently run on a single host with private addresses,
either over veth or by setting a nic in loopback mode with macvlan.

Support running between two real devices. Allow overriding addresses.

Also cut timeout to fail faster on error and explicitly log success.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>