fedcm-mojojs-helper.js exposes fedcm_mojo_mock_test which is a specialized promise_test which comes pre-setup with the appropriate mocking infrastructure to emulate platform federated auth backend. The mock is passed to the test function as the second parameter.
Example usage:
<script type="module">
import {fedcm_mojo_mock_test} from './support/fedcm-mojojs-helper.js';
fedcm_mojo_mock_test(async (t, mock) => {
mock.returnToken("https://idp.test/fedcm.json", "a_token");
assert_equals("a_token", await navigator.credentials.get(options));
}, "Successfully obtaining a token using mock.");
</script>
The chromium implementation uses the MojoJS shim.