Add documentation for Flux testing

[raz-iacob]

It would be good to have some documentation for testing. For example I wanted to test if a modal was closing and eventually I figure it out that I can do

->assertDispatched(
        event: 'modal-close', 
        name: 'add-new'
)

But there are many other cases where I have no idea how to test the Flux components.

What do you guys think?