This is a followup to CL 3676.

Rather than silently returning from the test, a pass,
use the Skip facility to mark the test as skipped.

Change-Id: I90d237e770150bf8d69f14fb09874e70894a7f86
Reviewed-on: https://go-review.googlesource.com/3682Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>