[BUGFIX beta] Only set pendingAjaxRequests once.

This was introduced in https://github.com/emberjs/ember.js/pull/4435, where we are setting `pendingAjaxRequests` everytime Ember.setupForTesting is called.  This causes assertions in an EAK style test suite where a new application is created for each test, since their might be a pending ajax request that was left over from a prior test.

/cc @mixonic