Improve testing support

[haroldadmin]

This PR makes the following changes:

• Add an internal drain method to VectorViewModel for draining all jobs launched using viewModelScope and its StateProcessor. This method is marked internal and is unavailable for use subclasses or consumers of the library
• Add a vector-test module
• Add a VectorTestRule for delegating the responsibility of setting the Main dispatcher to a regular dispatcher in unit tests
• Add a test-utility method to VectorTestRule to drain view models
• Updates documentation in various places

This PR needs more review and work. Marking it as a draft PR until then.