Allow tests to run over IPv6 as well as Legacy IP

Allow tests to run over IPv6 as well as Legacy IP

When run in an environment with no Legacy IP addresses, or no IPv6 addresses,
AI_ADDRCONFIG will cause getaddrinfo() not to return addresses of that type.

So when running in an IPv6-only environment, ocserv doesn't listen on Legacy
IP. And thus the tests fail. Fix this by using a hostname 'sockwrap' for the
test connections, and providing '--resolve' arguments for both the Legacy IP
and IPv6 addresses handled by libsocket_wrapper.

Some of the python test servers which don't use AI_ADDRCONFIG do still work
on Legacy IP, so leave those alone for now.

We recently added '-4' to the socat invocation for the nullppp tests, for
similar reasons (becaose socat started listening on IPv6 by default). We
can remove that now too.

Closes #721

Signed-off-by: David Woodhouse <dwmw2@infradead.org>