[DOCS] for sample: Unit testing of Cloud Functions offline test not working

[Berki2021]

Which sample?

Sample name or URL Link to closest heading https://firebase.google.com/docs/functions/unit-testing

What is the issue with this sample's docs?

Not working when copying 1 to 1. Always getting the following error when trying to do the offline sample: Error: The default Firebase app does not exist. Make sure you call initializeApp() before using any of the Firebase services.

Code not working