net/http/httputil: Made reverseproxy test less flaky.

net/http/httputil: Made reverseproxy test less flaky.

The reverseproxy test depended on the behavior of
runtime.NumGoroutines(), which makes no guarantee when
goroutines are reaped. Instead, modify the flushLoop()
to invoke a callback when it returns, so the exit
from the loop can be tested, instead of the number
of gorountines running.

R=bradfitz
CC=golang-dev
https://golang.org/cl/6068046