Clean up 'interfaces' smb.conf in selftest + remove unused client IPs

These are some small tidy-ups so that the 'interface' smb.conf setting for the testenvs is generated in one place. This keeps assumptions about the IP subnets in one place.

The main change is that the extra client.conf IPs have been removed, as they did not appear to be used. (The IPs are still reserved, if we need to simulate multiple different clients in future). The client.conf also now has an IPv6 address, to make it more consistent with the server smb.conf (although the client did not need this IPv6 address, as all the tests worked fine without it).