httptest: add NewUnstartedServer
httptest: add NewUnstartedServer

This allows testing TLS with different http.Server
options (timeouts, limits).

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5094043

wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old