Add documentation regarding testing

[frankie567]

Following discussion #672, we should add details in the documentation about how you can properly override the user dependency during testing.

[taylor-shift]

Is this the same as https://fastapi-users.github.io/fastapi-users/cookbook/create-user-programmatically/?

[frankie567]

@luisroel91 No, it will be slightly different.

The goal is to add some guidance to properly write unit tests with FastAPI Users, especially how to use the dependency_overrides mechanism of FastAPI to "mock" the current_user dependency.

I've added some insights here: #830 (comment)

[mixa2130]

@frankie567 added pr to show the example of testing fastapi-users