selftests: mptcp: connect: skip disconnect 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 full support of disconnections from the userspace
introduced by commit b29fcfb5 ("mptcp: full disconnect
implementation").

It is possible to look for "mptcp_pm_data_reset" in kallsyms because a
preparation patch added it to ease the introduction of the mentioned
feature.

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


Fixes: 05be5e27 ("selftests: mptcp: add disconnect tests")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>