avoid stopping event loop in app fixture

let pytest-asyncio do it

jupyterhub/app.py

@@ -3887,6 +3887,10 @@ class JupyterHub(Application):
             tasks = [t for t in asyncio.all_tasks()]
             for t in tasks:
                 self.log.debug("Task status: %s", t)
+        self._stop_event_loop()
+
+    def _stop_event_loop(self):
+        """In a method to allow tests to not do this"""
         asyncio.get_event_loop().stop()
 
     def stop(self):

jupyterhub/tests/mocking.py

@@ -382,6 +382,10 @@ class MockHub(JupyterHub):
         super().stop()
         self.db_file.close()
 
+    def _stop_event_loop(self):
+        # leave it to pytest-asyncio to stop the loop
+        pass
+
     async def login_user(self, name):
         """Login a user by name, returning her cookies."""
         base_url = public_url(self)