selftests/tcp_ao: Make RST tests less flaky

Currently, "active reset" cases are flaky, because select() is called
for 3 sockets, while only 2 are expected to receive RST.
The idea of the third socket was to get into request_sock_queue,
but the test mistakenly attempted to connect() after the listener
socket was shut down.

Repair this test, it's important to check the different kernel
code-paths for signing RST TCP-AO segments.

Fixes: c6df7b23 ("selftests/net: Add TCP-AO RST test")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>