Add ModelHelpers to TestFacade

Hello, thanks for this nice utility for testing! :)

I'm using it on Laravel and thought about integrating the ModelHelpers inside TestFacade to make easier to use the whole suite in a consistent way. So I'm proposing it here.

This PR add ModelHelper to TestFacade and adds all ModelHelpers method as aliases in Assert and Should, for better consistency.

Thanks for your work!
cheers