Mock Firebase Admin Auth for Unit Testing Authenticated Routes

Question

I'm using firebase-admin for authentication on my express backend. I have a middleware that checks if requests are authenticated.

public resolve(): (req, res, next) => void {
  return async (req, res, next) => {
    const header = req.header('Authorization');
    if (!header || !header.split(' ')) {
      throw new HttpException('Unauthorized', UNAUTHORIZED);
    }
    const token = header.split(' ')[1];
    await admin.auth().verifyIdToken(token).then((decodedToken: any) => {
      req.user = decodedToken;
      next();
    }).catch((error: any) => {
      throw new HttpException(error, UNAUTHORIZED);
    });
  };
}

So far, I can only unit test my routes to make sure that they respond UNAUTHORIZED instead of NOT_FOUND.

it('GET /api/menu should return 401 ', done => {
  const NOT_FOUND = 404;
  const UNAUTHORIZED = 401;
  supertest(instance)
    .get('/api/menu')
    .end((error, response: superagent.Response) => {
      expect(response.status).not.toEqual(NOT_FOUND);
      expect(response.status).toEqual(UNAUTHORIZED);
      done();
    });
});

But, I want to write more unit tests than this! I want to mock users so I can make AUTHORIZED requests! I want to use the type property I have on users to verify that users of a certain type can/cannot use certain routes. Does anyone have an idea of how I could do this with firebase-admin-node?

It looks like the firebase-admin-node repo generates tokens for unit tests here, but I'm not sure how I would apply that to my specific problem.