selftests/net: calibrate fq_band_pktlimit

This test validates per-band packet limits in FQ. Packets are dropped
rather than enqueued if the limit for their band is reached.

This test is timing sensitive. It queues packets in FQ with a future
delivery time to fill the qdisc.

The test failed in a virtual environment (vng). Increase the delays
to make it more tolerant to environments with timing variance.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>