go.net/ipv4: add missing per interface multicast listener tests
go.net/ipv4: add missing per interface multicast listener tests

Also removes unnecessary build tag.

R=dave
CC=golang-dev
https://golang.org/cl/21270043

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