Due to legacy/trac#21900, we need to explicitly specify `ServerDNSResolvConfFile /dev/null` for chutney to work on macOS when the network is unavailable. We should also set `ServerDNSDetectHijacking 0`. This should be the default, because: * users who run chutney might not want it using DNS in a detectable pattern, and * it makes chutney more reliable, because it no longer depends on a working DNS. Some users will want chutney to be able to use hostnames, so we should add a tools/test-network.sh option and environmental variable to re-enable the default ServerDNSResolvConfFile setting (or, even better, use a custom ServerDNSResolvConfFile). There seems to be no reason to turn on ServerDNSDetectHijacking: some users might be using chutney with internal DNS names.