courier: Use DNSError.IsNotFound to identify NXDOMAIN

When resolving MX records, we need to distinguish between "no such
domain" and other kinds of errors. Before Go 1.13, this was not
possible, so we had a workaround that assumed any permanent error was a
"no such domain", which is not great, but functional.

Now that our minimum supported version is Go 1.15, we can remove the
workaround.

This patch replaces the workaround with proper logic using
DNSError.IsNotFound to identify NXDOMAIN results when resolving MX
records.

This requires to adjust a few tests, that used to work on environments
where resolving unknown domains (used for testing) returned a permanent
error, and now they no longer do so. Instead of relying on this
environmental property, we make the affected tests use our own DNS
server, which should make them more hermetic and reproducible.

test/util/minidns.go

@@ -209,6 +209,11 @@ func (m *miniDNS) handle(msg *dnsmessage.Message) *dnsmessage.Message {
 			ID:       msg.ID,
 			Response: true,
 			RCode:    dnsmessage.RCodeSuccess,
+
+			// We're authoritative for the zones we're serving.
+			// We should either set this, or RecursionAvailable, otherwise
+			// some client libraries will complain.
+			Authoritative: true,
 		},
 		Questions: msg.Questions,
 	}