selftests: mptcp: improve 'fair usage on close' stability

The mentioned tests has to wait for a subflow creation failure.
The current code looks for TCP sockets in TW state and sometimes
misses the relevant event. Switch to a more stable check, looking
for the associated mib counter.

Fixes: 46e967d ("selftests: mptcp: add tests for subflow creation failure")
Closes: #257
Reported-and-tested-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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

@@ -1100,7 +1100,7 @@ wait_for_tw()
 	local ns=$1
 
 	while [ $time -lt $timeout_ms ]; do
-		local cnt=$(ip netns exec $ns ss -t state time-wait |wc -l)
+		local cnt=$(ip netns exec $ns nstat -as TcpAttemptFails | grep TcpAttemptFails | awk '{print $2}')
 
 		[ "$cnt" = 1 ] && return 1
 		time=$((time + 100))