selftests: mptcp: pm nl: also list skipped tests

If the feature is not supported by older kernels, and instead of just
ignoring some tests, we should mark them as skipped, so we can still
track them.

Fixes: d85555ac ("selftests: mptcp: pm_netlink: format subtests results in TAP")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>