mptcp: simult flow self-tests

Add a bunch of test-cases for multiple subflow xmit:
create multiple subflows simulating different links
condition via netem and verify that the msk is able
to use completely the aggregated bandwidth.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>