Fix test sporadically failing due to failure to return Promise

When this test was converted from CoffeeScript to JS, the implicit
return of the Promise was not converted to an explicit return. Hence the
test would sporadically fail depending on when the Promise callback got
executed.

src/sidebar/components/test/stream-content-test.js

@@ -122,7 +122,7 @@ describe('StreamContentController', function () {
 
     createController();
 
-    Promise.resolve().then(function () {
+    return Promise.resolve().then(function () {
       assert.calledOnce(fakeAnnotationMapper.loadAnnotations);
       assert.calledWith(fakeAnnotationMapper.loadAnnotations,
         ['annotation_1', 'annotation_2'], ['reply_1', 'reply_2', 'reply_3']);