We should maintain a test version of the backend for people writing new clients

For instance, running `make test` in `node-client` won't really work unless you have server software running locally. Until we decided what to do about open-sourcing the server, this would be a good workaround.

Start by reviewing the node-client setup and running make test to document its dependency on locally running server software. Clarify the required scope for a maintained test backend and how it should be exposed to client developers. Done means node-client tests can run against that backend without requiring separate local server software.

Written by the indexing model from the issue text.