[REQUIRED] Describe your environment

• Operating System version: MacOS 10.13.6
• Browser version: nodejs v8.11.2
• Firebase SDK version: firebase@^5.8.0:
• Firebase Product: auth

[REQUIRED] Describe the problem

This is related to #1487. When using firebase together with firebase-auth you need to call firebase.app().delete() and firebase.auth().signOut() so the node process can terminate properly (which is required if you e.g. use firebase with auth for tests in a CI environment).
This is not a bug per se, but I think it would be a nice feature if firebase.app().delete() would take care of cleaning up everything (or at least the things that prevent the node process from being terminated)

Steps to reproduce:

Relevant Code:

describe('', () => {
    let firebaseApp;

    before(async () => {
        firebaseApp = await firebase.initializeApp(config);
        return firebaseApp.auth().signInWithEmailAndPassword(mail, password);
    });

    after(() => {
        firebaseApp.auth().signOut(); // this is necessary so the process terminates
        firebaseApp.delete();
    });

    it('....', () => {
    });
});