selftests: mptcp: join: skip userspace PM tests if not supported

Selftests are supposed to run on any kernels, including the old ones not
supporting all MPTCP features.

One of them is the support of the userspace PM introduced by commit
4638de5a ("mptcp: handle local addrs announced by userspace PMs")
and the following ones.

It is possible to look for the MPTCP pm_type's sysctl knob to know in
advance if the userspace PM is available.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368


Fixes: 5ac1d2d6 ("selftests: mptcp: Add tests for userspace PM type")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>