[BUGFIX beta] Move AJAX listeners into Ember.setupForTesting.

Currently, if AJAX is being performed in an initializer the standard
`wait` helper will not be aware of it since the `Test.pendingAjaxRequests` is
initialized to 0 upon `App.injectTestHelpers`. This is AFTER the
application initializers have run.

This changes to allow calling `Ember.setupForTesting` before creating
your application instance (for environments like EAK that create a new
application for each test), and is completely backwards compatible.
Merge pull request #4435 from rjackson/move-ajax-listeners-to-ember-setup-for-testing

[BUGFIX beta] Move AJAX listeners into Ember.setupForTesting.