mptcp-cork: Fix some cork tests

As of now, it seems like using an RTO for cork is correct. So, to make the tests pass,
we should use this.

Also , in tcp_cork_on_05.pkt, remove the delay to send an ack, because it
triggers a loss-probe otherwise.

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>