While we're here, fix a Skip/Skipf error I noticed.

Fixes #19796.

(This fixes failures on the release branch introduced by cherry-pick
CL 39605.)

Change-Id: I59b1f5b5ea727fc314acfee8445b3de0b5af1e46
Reviewed-on: https://go-review.googlesource.com/39612
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>