Logf doesn't make the test fail, so the test was always OK.

Change-Id: I7c10ee74ff7e5d28cbd3a35e185093cb9f349470
Reviewed-on: https://go-review.googlesource.com/120496
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>