selftests: mptcp: join: one line for join check

Most tests are checking if the expected number of SYN/SYN+ACK/ACK JOINs
have been received, each of them on one line.

More Join related tests are going to be checked soon, no need to add 5
new lines per test in case of success, just one is enough. In case of
issue, the errors will still be reported like before.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240902-net-next-mptcp-mib-mpjtx-misc-v1-5-d3e0f3773b90@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>