Also include test for interface state (up or down).

Updates #13606

Change-Id: I03538d65525ddd9c2d0254761861c2df7fc5bd5a
Reviewed-on: https://go-review.googlesource.com/17850Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Run-TryBot: Russ Cox <rsc@golang.org>