selftests/net: add tests for PACKET_FANOUT_FLAG_UNIQUEID

Create two groups with PACKET_FANOUT_FLAG_UNIQUEID, add a socket to one.
Ensure that the groups can only be joined if all options are consistent
with the original except for this flag.

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