Mock the tests used for danetool
Currently danetool is being tested by the test suite using a set of servers known to provide valid or intentionally invalid answers. This prevents running the DANE testsuite on servers with broken DNSSEC setup or without any connectivity, in addition to making it unreliable (e.g., when one of the known servers is broken). We should be able to run that test suite without any connectivity at all, possibly based on socket_wrapper and resolv_wrapper.