selftests: mptcp: join: skip fail tests if not supported

selftests: mptcp: join: skip fail 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 the MP_FAIL / infinite mapping introduced
by commit 1e39e5a32ad7 ("mptcp: infinite mapping sending") and the
following ones.

It is possible to look for one of the infinite mapping counters to know
in advance if the this feature is available.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: b6e074e171bc ("selftests: mptcp: add infinite map testcase")
Cc: stable@vger.kernel.org
Fixes: 2ba18161d407 ("selftests: mptcp: add MP_FAIL reset testcase")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>