This reduces the wall time to run these benchmarks by about 30%.

Change-Id: I494a93c93e5acb1514510d85f65796f62e1629a5
Reviewed-on: https://go-review.googlesource.com/54650
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>