Tools like ZombieJS, (PhantomJS, and more) are built in order to run tests without a browser. And so they are called headless. They don't require a browser window to start, they don't show any interactions on screen.

They briefly mention the "delay" option that's been introduced to allow for manual delays to be put into your code. You can find out more about this testing tool from its main project page or by just jumping in and reading through the docs. (You can also find the full code over on github.)