Potential fix for flaky TeamDiscussionView js tests

Adding waitsFor check in the beforeEach method. The condition I'm
waiting for here is shamelessly borrowed from bok-choy's
wait_for_ajax._is_ajax_finished() method:
https://github.com/edx/bok-choy/blob/master/bok_choy/page_object.py#L431

This seems to fix the errors I was able to generate with the
set_timeout, which themselves looked ver similar to the failing
master tests, so I *think* this will fix our woes, but since I was
never able to get the tests to fail exactly the same way I can't be sure.

Since there don't seem to be any drawbacks, this is going into mainline
as-is and we'll monitor for the sporadic failures over the next few
weeks.

lms/djangoapps/teams/static/teams/js/spec/views/team_discussion_spec.js

@@ -8,6 +8,7 @@ define([
         var discussionView, createDiscussionView, createPost, expandReplies, postReply;
 
         beforeEach(function() {
+            runs(function() {
                 setFixtures('<div class="discussion-module""></div>');
                 $('.discussion-module').data('course-id', TeamSpecHelpers.testCourseID);
                 $('.discussion-module').data('discussion-id', TeamSpecHelpers.testTeamDiscussionID);
@@ -16,6 +17,13 @@ define([
                 DiscussionSpecHelper.setUnderscoreFixtures();
             });
             
+            waitsFor(
+                function() {return typeof(jQuery)!='undefined' && jQuery.active==0;},
+                "Ajax did not finish",
+                10000
+            })
+        });
+
         createDiscussionView = function(requests, threads) {
             discussionView = new TeamDiscussionView({
                 el: '.discussion-module'