go.net/ipv6: add missing per interface multicast listener test
go.net/ipv6: add missing per interface multicast listener test

R=golang-dev, dave
CC=golang-dev
https://codereview.appspot.com/15550043

polydawn-ports/go.net,gigq/go.net,go-mirrors/go.net,fernandez14/go.net,ssor/go.net