evdns adds default DNS servers, but chutney wants a consistent environment
When we test tor exits using chutney, we want a consistent network environment, so that tests pass or fail reliably.
This means that we want to be able to configure exits with no DNS servers, and have them only connect to IP addresses.
But evdns adds default name server(s) in some circumstances (for example, 127.0.0.1:53.)
This default could also be an issue on an IPv6-only system, where we'd want [::1]:53. (Yes, some IPv6-only systems don't even have an IPv4 localhost!)
This is also broken because of the empty/missing resolv.conf behaviour in #21900 (moved).