net/http: fix benchmark goroutine leak
net/http: fix benchmark goroutine leak

New detection because of net/http now using TestMain.

Fixes Issue 9033

LGTM=iant
R=golang-codereviews, iant
CC=adg, golang-codereviews, rsc
https://codereview.appspot.com/170210043

bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh