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

A new check is then added to make sure MPTCP is supported. If not, the
test stops and is marked as "skipped". Note that this check can also
mark the test as failed if 'SELFTESTS_MPTCP_LIB_EXPECT_ALL_FEATURES' env
var is set to 1: by doing that, we can make sure a test is not being
skipped by mistake.

A new shared file is added here to be able to re-used the same check in
the different selftests we have.

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


Fixes: 048d19d4 ("mptcp: add basic kselftest for mptcp")
Cc: stable@vger.kernel.org
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>