https://go-review.googlesource.com/#/c/27580 added the test.
However the test use syscall.ELOOP which is not defined on plan9.
Move test code from "os_test.go" to "os_windows_test.go" to prevent
build error.

Change-Id: Ie7f05bfb9ab229e06a8e82a4b3b8a7ca82d4663b
Reviewed-on: https://go-review.googlesource.com/27833
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>