Instead of a 10 second test unit, make it 13 sub-second ones. This
takes advantage of multiple builders better.

Fixes #12623

Change-Id: I3fb2eb02f899f25749e34b546b9d41b742a746cd
Reviewed-on: https://go-review.googlesource.com/14738Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>