Firebase cloud function testing with emulator instead of cloud - firebase

Question

The documentation for creating cloud function testing directs the user to configure the test suite.
Here's the code block it directs the user to create:
// At the top of test/index.test.js
const test = require('firebase-functions-test')({
databaseURL: 'https://my-project.firebaseio.com',
storageBucket: 'my-project.appspot.com',
projectId: 'my-project',
}, 'path/to/serviceAccountKey.json');
We wish to use online mode, but we wish to test against the local firebase emulator instead of a cloud instance. We attempted to initialize the test by omitting the key file and redirecting the database url as below:
require('firebase-functions-test')({
databaseURL: 'localhost:9000',
storageBucket: 'my-project.appspot.com',
projectId: 'my-project',
});
Unfortunately this does not work. The program will just throw the exception:
#firebase/database: FIREBASE WARNING: {"code":"app/invalid-credential","message":"Credential implementation provided to initializeApp() via the "credential" property failed to fetch a valid Google OAuth2 access token with the following error: "Error fetching access token: Error while making request: getaddrinfo ENOTFOUND metadata.google.internal. Error code: ENOTFOUND"."}
How is a user supposed to hookup to an emulator rather than a cloud instance?