Run integration tests on heroku using NodeJS

I'm using Heroku node buildpack to run my express HTTP server.

My HTTP server is used as a callback server

Some client triggers service A and this service talks to my HTTP server to get some data and return a response to the client.

In order to test it, my tests are running the server (using Supertest). They mimic the client which sends a request to service A, then this service A needs to get back to my server. And this way I complete a full loop.

The problem arises when the Heroku tests are running because I don't know where to point service A? Does Heroku prepare some temporary dyno I can trigger while the tests are running?

On my machine, I'm using ngrok to accomplish it.

I was thinking maybe Heroku exposes some URL in the process.env but found nothing interesting there (only a testId field)

Any idea how to run this kind of tests using heroku CI tests?