fix flaky test

the auto-reconnect test was flaky because we were
shutting down the server before we could be sure
the element had in fact been displayed

Author: rmorshea

tests/test_client/test_app.py

@@ -21,6 +21,8 @@ def OldComponent():
     with mount_point:
         mount_point.mount(OldComponent)
         driver.get(mount_point.url())
+        # ensure the element is displayed before stopping the server
+        driver.find_element_by_id("old-component")
 
     # the server is disconnected but the last view state is still shown
     driver.find_element_by_id("old-component")