[dev-hilt] Mocking AddEditTaskViewModel instead of using FakeRepository in AddEditTaskFragmentTest

All the UI tests in the dev-hilt branch are using a fake implementation of `TasksReporitory` to indirectly control the behavior of ViewModels. This approach works because the same instance of `FakeRepository` is injected into both `AddEditTaskFragmentTest` and `AddEditTaskViewModel`. In other words, `FakeRepository` is implemented as a singleton.

What I want to achieve is to mock the `AddEditTaskViewModel` instead of using `FakeRepository`. As `AddEditTaskViewModel` is private in `AddEditTaskFragment` there is no easy way to mock the AddEditTaskViewModel.

The closest thing I've found is in [this post](https://proandroiddev.com/testing-the-untestable-the-case-of-the-viewmodel-delegate-975c09160993), but IMHO the solution provided is not a clean one.

Start with AddEditTaskFragmentTest and AddEditTaskFragment in the dev-hilt branch, then inspect how AddEditTaskViewModel and the singleton FakeRepository are connected. Define a testable ViewModel boundary and update the UI tests to mock AddEditTaskViewModel instead of controlling it through FakeRepository; the issue does not specify the intended design.

Written by the indexing model from the issue text.