[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).

从围绕 AngularFireModule 和 provideFirebaseApp 的 AngularFire 7 测试设置开始，然后在 mock Firestore、Auth 和 Analytics 方法时重现报告的单元测试错误。完成的标准是记录一份在 Angular 12、Firebase 9 和 AngularFire 7 下 mock 这些 AngularFire 方法的可行指南。

由索引模型根据 Issue 内容生成。