selftests: mptcp: join: list failure at the end

With ~100 tests, it helps to have this summary at the end not to scroll
to find which one has failed.

It is especially interseting when looking at the output produced by the
CI where the kernel logs from the serial are mixed together.

Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>