Only run IPv6 tests if we have an IPv6 address

Only run IPv6 tests if we have an IPv6 address

* test/src/process-tests.el (ipv6-is-available): New function for
checking whether we have a globally routable IPv6 prefix assigned.
(lookup-family-specification): Use 'ipv6-is-available' to check for
IPv6.  Use 'localhost' instead of 'google.com' to test
'network-lookup-address-info' API.
(lookup-google): Use 'ipv6-is-available' to check for
IPv6.

* test/lisp/net/nsm-tests.el (nsm-ipv6-is-available): Rename to
'ipv6-is-available', make identical to the one in
test/src/process-tests.el.