Explain why... and what it does:

"The setupForTesting() function call makes ember turn off its automatic run loop execution. This gives us an ability to control the flow of the run loop ourselves, to a degree. Its default behaviour of resolving all promises and completing all async behaviour are suspended to give you a chance to set up state and make assertions in a known state. In other words, you know that if you run "visit" to get to a particular URL, you can be sure the URL has been visited and that's the only behaviour that has transpired. If we didn't use this mode, our assertions wouldn't be executed after the async behaviour had transpired, so our assertion results would be unpredictable."

Feel free to rewrite this!