DocumentReference.create does not exist when initializeAdminApp in @firebase/testing

### Describe your environment

* Operating System version: MacOS 10.14.4
* @firebase/testing: 0.9.4
* Firebase Product: testing / firestore

### Describe the problem

Because `@firebase/testing` uses the `firebase` (client) package to emulate the admin SDK the `DocumentReference.create` and `Transaction.create` methods don't exist when they do in the admin SDK.

#### Relevant Code:
```typescript
const mockFirebase = require('@firebase/testing')
const firebaseApp = mockFirebase.initializeAdminApp({})
const db = firebaseApp.firestore()
db.doc('test/1234').create({ a: 1 }) . // TypeError: db.doc(...).create is not a function
```