[docs] Mocking of angularfire methods with angularfire 7 during tests

### Version info

**Angular:** 12

**Firebase:** 9

**AngularFire:** 7

### How to reproduce these conditions

How to mock firebase operator during tests suits ?
How to write unit tests with mocks of firestore/auth/analytics methods ?

### Expected behavior

With Angularfire 6, I was able to mock angularfire methods like doc, collection and other methods to write my unit tests.
I would like to be able to mock it.

### Actual behavior

I am getting the error during my unit tests :
Error: Either AngularFireModule has not been provided in your AppModule (this can be done manually or implictly using provideFirebaseApp) or you're calling an AngularFire method outside of an NgModule (which is not supported).

AngularFireModule と provideFirebaseApp 周辺の AngularFire 7 のテスト設定から始め、Firestore、Auth、Analytics のメソッドをモックしながら、報告されたユニットテストのエラーを再現します。Angular 12、Firebase 9、AngularFire 7 でこれらの AngularFire メソッドをモックするための動作するガイダンスを文書化できれば完了です。

索引モデルが issue の本文から書いたものです。