Some multicast tests depend on proper IP routing because multicasting
requires an unicast source address for its delivery. We call a network
interface that can route IP traffic to neighbors a routed interface
conventionally. This CL makes sure that the interface under test is a
routed interface to avoid using non-routed network interfaces for IP
routing.

Also removes unnecessary build tag.

Fixes golang/go#6709.

R=dave
CC=golang-dev
https://golang.org/cl/21260043