go.net/netutil: fix unintentional test deadlock on resource starvation
go.net/netutil: fix unintentional test deadlock on resource starvation

It's been observed when the node under test has a tiny resource
configutation or package http has changed the behavior of its own
active connection pool.

For example,
http://build.golang.org/log/8912bc0944628cf1b4fe4063a77d36d19f9dd6a3

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/78640043

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