Also enable TestFstatat for all Unices now that they all implement
Fstatat.

Change-Id: I9722437042d5fc0df4a27d3a6a2be3d124c60057
Reviewed-on: https://go-review.googlesource.com/100617
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>