bytes: use Run method for benchmarks

Change-Id: I34ab1003099570f0ba511340e697a648de31d08a
Reviewed-on: https://go-review.googlesource.com/23427
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>