http2: fix racey and flaky server push test The bad test was added in https://go-review.googlesource.com/32887. My fault. Change-Id: Iee98ef0579bc5de086fa286530d45db009407d10 Reviewed-on: https://go-review.googlesource.com/32910 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org>