x/net/internal/nettest: make routableIP not return the ipv6 loopback address
x/net/internal/nettest: make routableIP not return the ipv6 loopback address

On some platform the IPv6 loopback address is not suitable to assign a name.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/165710043

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