os: fix tests for AIX

os: fix tests for AIX

This commits fixes tests for AIX inside os package.

"hostname" command on AIX returns "name.domain" and not only "name".
So, "hostname -s" must be called.

Change-Id: I75e193bcb6ad607ce54ad99aabbed9839012f707
Reviewed-on: https://go-review.googlesource.com/c/144537
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>