[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).

從圍繞 AngularFireModule 和 provideFirebaseApp 的 AngularFire 7 測試設定開始，接著在 mock Firestore、Auth 和 Analytics 方法時重現回報的單元測試錯誤。完成的標準是記錄一份在 Angular 12、Firebase 9 和 AngularFire 7 下 mock 這些 AngularFire 方法的可行指南。

由索引模型根據 Issue 內容生成。