This test was taking a long time, reduce its zealousness.

Change-Id: Ib824247b84b0039a9ec690f72336bef3738d4c44
Reviewed-on: https://go-review.googlesource.com/2502Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Minux Ma <minux@golang.org>