selftests/net: Add a test to validate behavior of rx timestamps

Validate the behavior of the combination of various timestamp socket
options, and ensure consistency across ip, udp, and tcp.

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>