context/ctxhttp: fix data race in tests
context/ctxhttp: fix data race in tests

Change-Id: If36de4de2c4fac6767aa5490e6f3d8c15531dda6
Reviewed-on: https://go-review.googlesource.com/20275
Reviewed-by: Dave Day <4cba7a21e1f1fc896442779dcb55aeda6df97dfe@golang.org>

albertjin/golang-net,albertjin/golang-net,golang/net