[FIX] mail: tests: correctly destroy with createDiscuss helper

Before this commit, when using createDiscuss and performing
discuss.destroy, the parent widget was not destroyed.

This is an issue, because the mocked environment is attached
to the parent widget of discuss. Any call to this.getSession()
is mocked by the addMockEnvironment, and this mocked behaviour
is removed only when the corresponding widget is destroyed.

This commit overrides discuss.destroy(), so that the parent
widget is also destroyed, so that it removes the side effect on
this.getSession() from the mocked environment.

addons/mail/static/tests/helpers/test_utils.js

@@ -77,6 +77,15 @@ function createDiscuss(params) {
     var selector = params.debug ? 'body' : '#qunit-fixture';
     discuss.appendTo($(selector));
 
+    // override 'destroy' of discuss so that it calls 'destroy' on the parent
+    // instead, which is the parent of discuss and the mockServer.
+    discuss.destroy = function () {
+        // remove the override to properly destroy discuss and its children
+        // when it will be called the second time (by its parent)
+        delete discuss.destroy;
+        parent.destroy();
+    };
+
     return discuss.call('chat_manager', 'isReady').then(function () {
         return discuss;
     });