selftests: mptcp: userspace pm: skip 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 MPTCP Userspace PM introduced by commit 4638de5a
("mptcp: handle local addrs announced by userspace PMs").

We can skip all these tests if the feature is not supported simply by
looking for the MPTCP pm_type's sysctl knob.

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


Fixes: 259a834f ("selftests: mptcp: functional tests for the 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>