selftests: mptcp: join: skip Fastclose 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 MP_FASTCLOSE introduced in commit
f284c0c7 ("mptcp: implement fastclose xmit path").

If the MIB counter is not available, the test cannot be verified and the
behaviour will not be the expected one. So we can skip the test if the
counter is missing.

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


Fixes: 01542c9b ("selftests: mptcp: add fastclose testcase")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>