net-test: packetdrill: read outbound packets from tun device

In local mode, read the outbound packets from the tun device.

A recent Linux patch "tcp: avoid retransmits of TCP packets hanging in
host queues" means that TCP behavior now depends on whether and when
the network device (tun device here) consumes the packets. We now read
all these packets so that the kernel can exercise its normal code
paths for packet transmit completion.

An alternative approach would be to set the tun txqueuelen to 0, but
this would impact qdisc behavior. It is more realistic, and has fewer
side-effects, to make sure the network device has a non-zero queue
length, but packets are still consumed.

Change-Id: Ia922994d4eeea12e61d7876b4c4ef34bc9cef34e