selftests: mptcp: userspace pm: uniform verify events

Simply adding a "sleep" before checking something is usually not a good
idea because the time that has been picked can not be enough or too
much. The best is to wait for events with a timeout.

In this selftest, 'sleep 0.5' is used more than 40 times. It is always
used before calling a 'verify_*' function except for this
verify_listener_events which has been added later.

At the end, using all these 'sleep 0.5' seems to work: the slow CIs
don't complain so far. Also because it doesn't take too much time, we
can just add two more 'sleep 0.5' to uniform what is done before calling
a 'verify_*' function. For the same reasons, we can also delay a bigger
refactoring to replace all these 'sleep 0.5' by functions waiting for
events instead of waiting for a fix time and hope for the best.

Fixes: 6c73008aa3 ("selftests: mptcp: listener test for userspace PM")
Cc: stable@vger.kernel.org
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -913,6 +913,7 @@ test_listener()
 		$client4_port > /dev/null 2>&1 &
 	local listener_pid=$!
 
+	sleep 0.5
 	verify_listener_events $client_evts $LISTENER_CREATED $AF_INET 10.0.2.2 $client4_port
 
 	# ADD_ADDR from client to server machine reusing the subflow port
@@ -928,6 +929,7 @@ test_listener()
 	# Delete the listener from the client ns, if one was created
 	kill_wait $listener_pid
 
+	sleep 0.5
 	verify_listener_events $client_evts $LISTENER_CLOSED $AF_INET 10.0.2.2 $client4_port
 }