selftests: mptcp: diag: skip listen tests if not supported

selftests: mptcp: diag: skip listen 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 listen diag dump support introduced by
commit 4fa39b701ce9 ("mptcp: listen diag dump support").

It looks like there is no good pre-check to do here, i.e. dedicated
function available in kallsyms. Instead, we try to get info if nothing
is returned, the test is marked as skipped.

That's not ideal because something could be wrong with the feature and
instead of reporting an error, the test could be marked as skipped. If
we know in advanced that the feature is supposed to be supported, the
tester can set SELFTESTS_MPTCP_LIB_EXPECT_ALL_FEATURES env var to 1: in
this case the test will report an error instead of marking the test as
skipped if nothing is returned.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: f2ae0fa68e28 ("selftests/mptcp: add diag listen tests")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>