Add a grpc_ipv6_loopback_available() test utility, and only run IPv6-specific

tests on machines that support AF_INET6 sockets bound to ::1.

Listening on :: or connecting to a mapped address should always work.
	Change on 2014/12/10 by pmarks <pmarks@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=81837056