Fixes #11919

Issue #11918 suggested that os.File.Chown and os.Lchown were under tested.

Change-Id: Ib41f7cb2d2fe0066d2ccb4d1bdabe1795efe80fc
Reviewed-on: https://go-review.googlesource.com/12834Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>