How to create/install ember-cli test helpers with Ember 3?

Before v3.0, it used to be that to make a helper available to testing code, you'd import it into tests/helpers/start-app.js. Now that the testing reorg with 3.0 has removed that file, where should those imports go?

(FWIW the ember-cli docs still show the old, non-existent file: https://ember-cli.com/testing#single-helper-per-file )