Names of sub-benchmarks are preserved, short of the additional slash.

Change-Id: I9b3f82964f9a44b0d28724413320afd091ed3106
Reviewed-on: https://go-review.googlesource.com/23425Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>