Test ping to "all IPv6 nodes" multicast address

Test ping to "all IPv6 nodes" multicast address

Any Echo Response packet uses a different source address than
used for the target, because it is not allowed to use a multicast
address as source.  This results in extra output in a specific
format on standard error.

This should work, because IPv6 requires multicast support, and
every IPv6 node is supposed to join the "all nodes" multicast
group, including the node the test runs on.  It at least works
on my Ubuntu 20.04 LTS system.