selftests: mptcp: dump more info on mpjoin errors

Very occasionally, MPTCP selftests fail. Yeah, I saw that at least once!

Here we provide more details in case of errors with mptcp_join.sh script
like it was done with mptcp_connect.sh, see
commit 767389c8 ("selftests: mptcp: dump more info on errors")

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