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

One of them is the new listener events linked to the path-manager
introduced by commit f8c9dfbd ("mptcp: add pm listener events").

It is possible to look for "mptcp_event_pm_listener" in kallsyms to know
in advance if the kernel supports this feature and skip these sub-tests
if the feature is not supported.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 6c73008a ("selftests: mptcp: listener test for userspace PM")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>