Instead of comparing against the entire output that may include
verbose warning messages, use the last line of the output and check
it includes the expected success message (PASS).

Change-Id: Iafd583ee5529a8aef5439b9f1f6ce0185e4b1331
Reviewed-on: https://go-review.googlesource.com/9304Reviewed-by: David Crawshaw <crawshaw@golang.org>