Using MySQL on Heroku CI

I have a Rails app that uses a MySQL database. I'd like to use Heroku CI because of its close integration with pipelines, but I see the following error when setting up the test database:

PG::ConnectionBad: could not connect to server: Connection refused

Is the server running on host "127.0.0.1" and accepting TCP/IP connections on port 5432?

It's assuming that I'm using Postgres. I'm using the JawsDB addon for using MySQL on Heroku, so I tried to specify that for my test environment in my app.json file:

"environments": {
    "test": {
      "addons": [
        "jawsdb:kitefin"
      ]
    }
  },

But that didn't seem to make a difference and I see the same error. Does Heroku force you to use Postgres in CI? If so, I'll have to use another service like CircleCI.