Also add a test now that all unices have Renameat.

Change-Id: I9098662569e9910122dc686559bbd04be06328fa
Reviewed-on: https://go-review.googlesource.com/c/157898
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>