avoid using (external) network during tests

[Brian Warner]

There's a ​debian bug against Foolscap for using the external network during the build process (which is against debian policy, since builds should be deterministic, and therefore not influenced by the outside world).

I think this is happening because one of the tests tries to reverse-DNS a name, or tries to ping the nameservers to determine which interface is pointing to the outside world. We should fix this.

This is pretty important, because the debian bug has foolscap scheduled for removal from debian altogether.