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>
1 file changed