I wanted to change this test so it tries connections on both IPv4 &
IPv6. Having it connect on both is the easy bit. Harder was making
it not fail on machines that don't have IPv6 stack (or IPv4 in some
rare cases). TBH I wasn't able to test this, but it seems like this
should work.
In the end we want to modify nbdkit so it can listen on only certain
interfaces, but that's a bit more complicated.
Rich.