selftests: mptcp: connect: skip TFO 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 support of TCP_FASTOPEN socket option with MPTCP
connections introduced by commit 4ffb0a02 ("mptcp: add TCP_FASTOPEN
sock option").

It is possible to look for "mptcp_fastopen_" in kallsyms to know if the
feature is supported or not.

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


Fixes: ca7ae891 ("selftests: mptcp: mptfo Initiator/Listener")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>