mptcp: more stable diag self-tests

During diag self-tests we introduce long wait in the mptcp test
program to give the script enough time to access the sockets
dump.

Such wait is introduced after shutting down one sockets end. Since
commit 43b54c6e ("mptcp: Use full MPTCP-level disconnect state
machine") if both sides shutdown the socket is correctly transitioned
into CLOSED status.

As a side effect some sockets are not dumped via the diag interface,
because the socket state (CLOSED) does not match the default filter, and
this cause self-tests instability.

Address the issue moving the above mentioned wait before shutting
down the socket.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/68
Fixes: df62f2ec

 ("selftests/mptcp: add diag interface tests")
Tested-and-acked-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>