FR: make node process terminateable after calling app.delete()

### [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:

```javascript
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('....', () => {
});
});
```