Revert "ipv6: disable multicast packet delivery tests on darwin 12 or below"

Looks like using IPv6 options on Darwin 12 kernel is a hard task.

This reverts commit 67a4d4ecbcb395bd234838c7ea5d8a61008ff215.

Change-Id: I1436e4ff7b42f068e09671ebe9c244a641eefdae
Reviewed-on: https://go-review.googlesource.com/33173
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>