selftests/net: Amend per-netns counter checks

selftests/net: Amend per-netns counter checks

Selftests here check not only that connect()/accept() for
TCP-AO/TCP-MD5/non-signed-TCP combinations do/don't establish
connections, but also counters: those are per-AO-key, per-socket and
per-netns.

The counters are checked on the server's side, as the server listener
has TCP-AO/TCP-MD5/no keys for different peers. All tests run in
the same namespaces with the same veth pair, created in test_init().

After close() in both client and server, the sides go through
the regular FIN/ACK + FIN/ACK sequence, which goes in the background.
If the selftest has already started a new testing scenario, read
per-netns counters - it may fail in the end iff it doesn't expect
the TCPAOGood per-netns counters go up during the test.

Let's just kill both TCP-AO sides - that will avoid any asynchronous
background TCP-AO segments going to either sides.

Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/all/20240201132153.4d68f45e@kernel.org/T/#u
Fixes: 6f0c472a6815 ("selftests/net: Add TCP-AO + TCP-MD5 + no sign listen socket tests")
Signed-off-by: Dmitry Safonov <dima@arista.com>
Link: https://lore.kernel.org/r/20240202-unsigned-md5-netns-counters-v1-1-8b90c37c0566@arista.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>