Usually this test is skipped (on builders and when not root), so
people are unlikely to see this error.

Updates #19296

Change-Id: I3acb81260034dad8776c305f83d7cbac4b718e75
Reviewed-on: https://go-review.googlesource.com/48191
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>