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

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

The commit 4656d72c1efa ("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 1cc94ac1af4b ("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: f59549814a64 ("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>