It was flaky on slower machines.

Per report at https://github.com/golang/go/issues/23399#issuecomment-405792381

Change-Id: I7cab02821f78b5ce02ea51089d7eb51723f9705f
Reviewed-on: https://go-review.googlesource.com/124835
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>