Remove network dependencies from unit tests
Some of the new unit tests in 0.2.8 (#17076 (moved)) seem to be using the resolver on non-existent addresses. This causes them to be slow when the network is up, but fast when it's down (at least on my OS X box).
mikeperry has also complained that they're slow on IRC.
We could mock the address resolution functions to always return NXDOMAIN during these tests to make them faster.