selftests: mptcp: fix diag instability

Instead of waiting for an arbitrary amount of time for the MPTCP
MP_CAPABLE handshake to complete, explicitly wait for the relevant
socket to enter into the established status.

Additionally let the data transfer application use the slowest
transfer mode available (-r), to cope with very slow host, or
high jitter caused by hosting VMs.

Fixes: df62f2ec ("selftests/mptcp: add diag interface tests")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/258


Reported-and-tested-by: Matthieu Baerts <matthieu.baerts@tessares.net>
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>