selftests: mptcp: dump more info on errors

Even if that may sound completely unlikely, the mptcp implementation
is not perfect, yet.

When the self-tests report an error we usually need more information
of what the scripts currently report. iproute allow provides
some additional goodies since a few releases, let's dump them.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>