selftests: mptcp: userspace: fix v4-v6 test in v6.1

The commit 4656d72c1e ("selftests: mptcp: userspace: validate v4-v6 subflows mix")
has been backported to v6.1.8 without any conflicts. But it looks like
it was depending on a previous one:

  commit 1cc94ac1af ("selftests: mptcp: make evts global in userspace_pm")

Without it, the test fails with:

  ./userspace_pm.sh: line 788: : No such file or directory
  # ADD_ADDR4 id:14 10.0.2.1 (ns1) => ns2, reuse port        [FAIL]
  sed: can't read : No such file or directory

This dependence refactors the way the monitoring files are being
created: only once for all the different sub-tests instead of per
sub-test.

It is probably better to avoid backporting the refactoring. That is why
the new sub-test has been adapted to work using the previous way that is
still in place here in v6.1: the monitoring is started at the beginning
of each sub-test and the created file is removed at the end.

Fixes: f59549814a ("selftests: mptcp: userspace: validate v4-v6 subflows mix")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>

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

@@ -778,6 +778,14 @@ test_subflows()
 
 test_subflows_v4_v6_mix()
 {
+	local client_evts
+	client_evts=$(mktemp)
+	# Capture events on the network namespace running the client
+	:>"$client_evts"
+	ip netns exec "$ns2" ./pm_nl_ctl events >> "$client_evts" 2>&1 &
+	evts_pid=$!
+	sleep 0.5
+
 	# Attempt to add a listener at 10.0.2.1:<subflow-port>
 	ip netns exec "$ns1" ./pm_nl_ctl listen 10.0.2.1\
 	   $app6_port > /dev/null 2>&1 &
@@ -820,6 +828,9 @@ test_subflows_v4_v6_mix()
 	ip netns exec "$ns1" ./pm_nl_ctl rem id $server_addr_id token\
 	   "$server6_token" > /dev/null 2>&1
 	sleep 0.5
+
+	kill_wait $evts_pid
+	rm -f "$client_evts"
 }
 
 test_prio()