Also retightens test cases for Resolve{TCP,UDP,IP}Addr which are using
interface names for specifying IPv6 zone.

Updates #14037.
Fixes #18362.

Change-Id: I7444b6302e2847dfbdab8a0ad5b2e702bed1a3d6
Reviewed-on: https://go-review.googlesource.com/34670
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>