selftests/net: Synchronize client/server before counters checks

selftests/net: Synchronize client/server before counters checks

On tests that are expecting failure the timeout value is
TEST_RETRANSMIT_SEC == 1 second. Which is big enough for most of devices
under tests. But on a particularly slow machine/VM, 1 second might be
not enough for another thread to be scheduled and attempt to connect().
It is not a problem for tests that expect connect() to succeed as
the timeout value for them (TEST_TIMEOUT_SEC) is intentionally bigger.

One obvious way to solve this would be to increase TEST_RETRANSMIT_SEC.
But as all tests would increase the timeouts, that's going to sum up.

But here is less obvious way that keeps timeouts for expected connect()
failures low: just synchronize the two threads, which will assure that
before counter checks the other thread got a chance to run and timeout
on connect(). The expected increase of the related counter for listen()
socket will yet test the expected failure.

Never happens on my machine, but I suppose the majority of netdev's
connect-deny-* flakes [1] are caused by this.

Prevents the following testing issue:
> # selftests: net/tcp_ao: connect-deny_ipv6
> # 1..21
> # # 462[lib/setup.c:243] rand seed 1720905426
> # TAP version 13
> # ok 1 Non-AO server + AO client
> # not ok 2 Non-AO server + AO client: TCPAOKeyNotFound counter did not increase: 0 <= 0
> # ok 3 AO server + Non-AO client
> # ok 4 AO server + Non-AO client: counter TCPAORequired increased 0 => 1
...

[1]: https://netdev-3.bots.linux.dev/vmksft-tcp-ao/results/681741/6-connect-deny-ipv6/stdout
Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://patch.msgid.link/20240823-tcp-ao-selftests-upd-6-12-v4-7-05623636fe8c@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>