Use t.Fatalf instead of t.Errorf followed by t.FailNow.

Change-Id: Ie31f8006e7d9daca7f59bf6f0d5ae688222be486
Reviewed-on: https://go-review.googlesource.com/c/144111
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>