[release-branch.go1.8] net: skip Windows test using getmac if getmac cmdlet not available

This doesn't appear to be present on Windows Server 2012 or 2016:

https://build.golang.org/log/6ea21b99c9b8a2be20f9aeaec6c425b84faf1af7
https://build.golang.org/log/2bcf04f1df003577352f4f987a39a59a081094ee

Updates golang/go#17513
Updates golang/go#20073

Change-Id: I72820704b4cb16bb1720b7f6a9f2e10028c71334
Reviewed-on: https://go-review.googlesource.com/41395


Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-on: https://go-review.googlesource.com/70845
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>