selftests: mptcp: wait for port instead of sleep

selftests: mptcp: wait for port instead of sleep

After having started mptcp_connect in listening mode,
'mptcp_lib_wait_local_port_listen' can be used to wait for the listening
socket to be ready.

This is better than using the 'sleep' command, not to pause for a fixed
amount of time, but waiting for an event. This helper is used in all
other MPTCP selftests, but not in these two.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251114-net-next-mptcp-sft-count-cache-stats-timeout-v1-7-863cb04e1b7b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>