make-test-network-all has never detected IPv6 on linux

When I wrote make-test-network-all, I tested IPv6 on OS X and BSD. But it's never worked on Linux.

On BSD and OS X systems, we use: ping6 -q -c 1 -o ::1

On Linux systems, we should use: ping6 -q -c 1 -W 1 ::1

You'd think we could get away with: ping6 -q -c 1 ::1 but BSD / OS X would hang forever on systems where IPv6 packets are dropped, and Linux is somewhat ambiguous about what happens in that case.

Over the long term, implementing checks like this in chutney might be a better idea.