Adds a test to detect the bug that slipped in earlier when partioning
by the Aux value, but not sorting by it.

Change-Id: I56d0ba76383bbc1514b3dabd295e369771c26645
Reviewed-on: https://go-review.googlesource.com/19382
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>