TestInterfaceHardwareAddrWithGetmac was panicing when getmac returned
multiple network cards.

Change-Id: I6fefa5a4910bce805b4cd9c09f94bd56c9682b9e
Reviewed-on: https://go-review.googlesource.com/47190Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>