toExist doesn't work

[jackocnr]

I was just struggling with toExist, when I came across the following issue/pull request, which I can now see has broken the functionality for me (and another commenter on that issue): #64

You can no longer test if an element exists unless you actually inject it into the DOM (of the spec page), which doesn't make sense to me. It's also not consistent with all the other tests.

I think we should revert that change, and maybe create a different function for toExistInDom, like the author of that issue suggested at the time.