go.net/ipv4: make sure whether the interface under test is routed
go.net/ipv4: make sure whether the interface under test is routed

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

kiip/net,raboof/net,gomini/net,gomini/net,cesanta/goxnet,flowlo/go-net,muzining/net,w4ngyi/net,dafyddcrosby/net,albertjin/golang-net,albertjin/golang-net,kiip/net,flowlo/go-net,zabawaba99/net,dafyddcrosby/net,yangxianjun/net,cesanta/goxnet,muzining/net,w4ngyi/net,foreversmart/net,yangxianjun/net,mckinney-subgraph/net,cmars/tools,mckinney-subgraph/net,raboof/net,veliamsli/net,cmars/tools,cmars/tools,golang/net,DramaFever/golang.org-x-net,DramaFever/golang.org-x-net,veliamsli/net,chenzlabs/net,cmars/tools,chenzlabs/net,foreversmart/net,subuk/net,zabawaba99/net,subuk/net