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 <[e-mail]>
Signed-off-by: Matthieu Baerts (NGI0) <[e-mail]>
Link: https://patch.msgid.link/2025111[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/mptcp/mptcp_sockopt.sh

···
 				${local_addr} < "$sin" > "$sout" &
 	local spid=$!
 
-	sleep 1
+	mptcp_lib_wait_local_port_listen "${listener_ns}" "${port}"
 
 	timeout ${timeout_test} \
 		ip netns exec ${connector_ns} \

tools/testing/selftests/net/mptcp/userspace_pm.sh

···
 	ip netns exec "$ns1" \
 	   ./mptcp_connect -s MPTCP -w 300 -p $app_port -l $listen_addr > /dev/null 2>&1 &
 	local server_pid=$!
-	sleep 0.5
+
+	mptcp_lib_wait_local_port_listen "${ns1}" "${port}"
 
 	# Run the client, transfer $file and stay connected to the server
 	# to conduct tests