go.net/ipv6: add ICMP checksum test

This CL enables testing raw ICMP packets with and without a setting of
ICMP checksum field.

Actually it's just for Linux because almost all BSD variants never
allow us to touch the checksum field of ICMP but Linux does when we use
the value 255 for the socket level.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/15020047
3 files changed