@firebase/testing doesn't return firestore() instance when firebase-admin is a dependency

### Environment

* Operating System version: macOS 10.14.4
* Browser version: Safari 12.1
* Firebase SDK version: @firebase/testing 0.7.9 through 0.9.4
* Firebase Product: @firebase/testing and firebase-admin

### Description
This is a refinement and isolation of issue #1630 specifically related to firebase-admin breaking @firebase/testing. Even if no code is called in firebase-admin the issue still presents.

`@firebase/testing` doesn't return a `firestore()` instance when `firebase-admin` is a dependency in `package.json`.

Steps to reproduce
1. `git clone https://github.com/firebase/quickstart-nodejs.git`
2. `cd quickstart-nodejs/firestore-emulator/javascript-quickstart/`
3. add `"firebase-admin": "7.3.0"` to `"dependencies"` in package.json. __This is the step which reproduces the issue__
4. `npm install`
5. `firebase serve --only firestore`

Now in the node the run the following code:
`const firebase = require("@firebase/testing");`
`firebase.firestore()`

This will produce an error:
`Thrown:`
`TypeError: firebase.firestore is not a function`