This change also uses nettest.SupportsIPv6MulticastDeliveryOnLoopback
to disable multicast packet delivery tests on some platform that doesn't
support the feature.

Fixes golang/go#17015.

Change-Id: Ia547efc2ac3779a96592bbc3874533fb65b711af
Reviewed-on: https://go-review.googlesource.com/33251
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>