[BUGFIX] Do not attach new listeners on each setupForTesting call

The test explains the issue pretty clearly, but for a more practical example, using EAK and upgrading to master caused all these errors to pop up:

![](https://api.monosnap.com/image/download?id=BVr5FUGdUg2uhinvKVfK9Pyn807Thu)

The `ajaxComplete` hook is added each time you call `setupForTesting`, and in EAK you call this function for each `startApp` call. I'm not clear on why tracking pending requests this way is preferable to the old style, but regardless this brings back the beta API behavior.

/cc @rjackson