Future-proof our use of Protobufs in tests

We currently exercise protobuf-related logic in our tests in ways it's not recommended by upstream. First, we use require.Equal that compares Protobuf messages via deep inspection. Second, we should embed resolver.ClientConn into our own test client connection. Both will cause our code to fail as soon as we migrate to protobuf-gen-go v1.4. So let's future-proof our code to work correctly with newer versions in case we decide to upgrade.