selftests: mptcp: simult flows: skip if MPTCP is not supported

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".

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 1a418cb8

 ("mptcp: simult flow self-tests")
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>