The dial-time SSRF guard runs on the resolved IP, so the tests that prove a
symbolic hostname cannot reach loopback used "localhost." — a trailing dot never
matches /etc/hosts, so Go queries real DNS. Machines whose resolver does not
answer "localhost." (a VPN DNS, for example) got "no such host" before the dial
guard ever ran, failing three specs.
Add tests.StubResolver, a net.Resolver backed by an in-memory DNS responder over
net.Pipe, and let the plugin dialers take a resolver so tests can inject it.
Name resolution in those specs no longer depends on the machine's DNS.