test: Add a quick test to see if newaddr over grpc-web-proxy works

[cdecker]

@ShahanaFarooqui reports that the newaddr method returns a not
implemented. This is my attempt to reproduce the issue,
unsuccessfully. I wonder if this is a mixup in the interfaces: we
might accidentally be talking to the scheduler rather than the
node. Any grpc server will return an unimplemented error for
methods that are not in their schema, hence a mixup could cause this.