Improve reliability of run_app test (#8309)

(cherry picked from commit c29945a)

tests/test_run_app.py

@@ -916,10 +916,16 @@ async def stop(self, request: web.Request) -> web.Response:
 
     def run_app(self, port: int, timeout: int, task, extra_test=None) -> asyncio.Task:
         async def test() -> None:
-            await asyncio.sleep(1)
+            await asyncio.sleep(0.5)
             async with ClientSession() as sess:
-                async with sess.get(f"http://localhost:{port}/"):
-                    pass
+                for _ in range(5):  # pragma: no cover
+                    try:
+                        async with sess.get(f"http://localhost:{port}/"):
+                            pass
+                    except ClientConnectorError:
+                        await asyncio.sleep(0.5)
+                    else:
+                        break
                 async with sess.get(f"http://localhost:{port}/stop"):
                     pass