[FIX] mail: test utils: mock is_odoo_focused

In the mocked version of the longpolling bus used in tests, the
is_odoo_focused function should be defined to prevent crashes when
we simulate incoming messages.

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

@@ -37,6 +37,9 @@ function createBusService(bus) {
                  * Do nothing
                  */
                 start_polling: function () {},
+                is_odoo_focused: function () {
+                    return true;
+                },
             });
         },