selftests/net: Fetch and check TCP-MD5 counters

selftests/net: Fetch and check TCP-MD5 counters

There are related TCP-MD5 <=> TCP and TCP-MD5 <=> TCP-AO tests
that can benefit from checking the related counters, not only from
validating operations timeouts.

It also prepares the code for introduction of mixed select()+poll mode,
see the follow-up patches.

Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://patch.msgid.link/20250319-tcp-ao-selftests-polling-v2-3-da48040153d1@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>