This will enable test sharding over multiple VMs, to speed trybot answers.

Update #10029

Change-Id: Ie277c6459bc38005e4d6af14d22effeaa0a4667e
Reviewed-on: https://go-review.googlesource.com/6531Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>