selftests/net: Add TCP-AO selfconnect/simultaneous connect test

Check that a rare functionality of TCP named self-connect works with
TCP-AO. This "under the cover" also checks TCP simultaneous connect
(TCP_SYN_RECV socket state), which would be harder to check other ways.

In order to verify that it's indeed TCP simultaneous connect, check
the counters TCPChallengeACK and TCPSYNChallenge.

Sample of the output:
> # ./self-connect_ipv6
> 1..4
> # 1738[lib/setup.c:254] rand seed 1696451931
> TAP version 13
> ok 1 self-connect(same keyids): connect TCPAOGood 0 => 24
> ok 2 self-connect(different keyids): connect TCPAOGood 26 => 50
> ok 3 self-connect(restore): connect TCPAOGood 52 => 97
> ok 4 self-connect(restore, different keyids): connect TCPAOGood 99 => 144
> # Totals: pass:4 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>