Unit test conditionally-set root page

I have trouble passing the unit test that I wrote for testing whether the root page is set correctly. The issue is that comp.rootPage is always undefined when the test executes. I know this is because that the test executes even before the root page is set in the component’s constructor, but I’m not sure how to force the test to wait until the callback of platform.ready() finishes executing.

This is what the constructor of my root component looks like:
// 1. during testing, constructor is entered first

  constructor(platform: Platform, storage: NativeStorageWrapper) {
    platform.ready().then(() => {
      // Okay, so the platform is ready and our plugins are available.
      // Here you can do any higher level native things you might need.

        data => this.rootPage = data ? LocationPage : MenuPage,
        error => this.rootPage = LocationPage

And the unit test:
// 2. then the expect line is executed without even executing the body of the ‘then’ in the constructor

it('should open location confirmation page for the 1st time', async(() => {
    fixture.whenStable().then(() => {

Exactly same problem here, anybody has a solution plz ?