selftests: mptcp: allow more slack for slow test-case

A test-case is frequently failing on some extremely slow VMs.
The mptcp transfer completes before the script is able to do
all the required PM manipulation.

Address the issue in the simplest possible way, making the
transfer even more slow.

Additionally dump more info in case of failures, to help debugging
similar problems in the future and init dump_stats var.

Fixes: e274f715

 ("selftests: mptcp: add subflow limits test-cases")
Cc: stable@vger.kernel.org
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/323
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>