Re-work the test to use wmic instead of PowerShell's getmac that's
only avaliable on Server 2008. Maintains duplicate detection added
for golang/go#21027.

Tested on windows-amd64-{2008, 2012, 2016} buildlets.
Enabling for Windows XP because it should work[1].

Fixes golang/go#20073

[1] https://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/wmic_overview.mspx?mfr=true

Change-Id: Ic11d569f7964f61d08ae0dcc1b926efc5336ac5b
Reviewed-on: https://go-review.googlesource.com/82975Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>