tests: test vnet_calc_checksums_inline(...)

tests: test vnet_calc_checksums_inline(...)

Use a hand-crafted packet generator script to inject packets with
deliberately broken ip4 and udp checksums - and the appropriate
checksum offload flags - into src/vnet/interface_output.c
vnet_interface_output_node_inline(...), and make sure that the
resulting checksums are correct.

Use the packet tracer to catch one of the packets. Parse through the
packet trace, and make sure that the checksums have been correctly
computed.

Type: test

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: Ic08b4b64d7ef27061558a3abf0b79917ead254b5