firebase functions: Unit test for storage onFinalize()

How can I write a unit test for the following firebase function:

functions.storage.object().onFinalize(object => {
return storage
      .bucket()
      .file(object.name)
      .delete()
      .then(() => console.log(`Deletion successful: '${object.name}'`))
      .catch(err => console.error(err));
);

I read the docs and setup a unit test for the 'online mode', connecting to a test project.

My approach is to write an assertion whether "Deletion successful" has been logged. I think the current unit test fails silently (I dont see any errors besides my failing assertion) because the provided object.name does not really exists so the file cant be found.

Do I have to create and upload this file before the test/ how can I mock it?