[5.0.x] Refs #35059 -- Used asyncio.Event in ASGITest.test_asyncio_cancel_error to enforce specific interleaving.

Sleep call leads to a hard to trace error in CI. Using an Event is
more deterministic, and should be less prone to environment
variations.

Bug in 11393ab1316f973c5fbb534305750740d909b4e4.

Backport of f4a08b6ddfcacadfe9ff8364bf1c6c54f5dd370f from main.

tests/asgi/tests.py

@@ -477,6 +477,7 @@ class ASGITest(SimpleTestCase):
         sync_waiter.active_threads.clear()
 
     async def test_asyncio_cancel_error(self):
+        view_started = asyncio.Event()
         # Flag to check if the view was cancelled.
         view_did_cancel = False
         # Track request_finished signal.
@@ -486,9 +487,10 @@ class ASGITest(SimpleTestCase):
 
         # A view that will listen for the cancelled error.
         async def view(request):
-            nonlocal view_did_cancel
+            nonlocal view_started, view_did_cancel
+            view_started.set()
             try:
-                await asyncio.sleep(0.2)
+                await asyncio.sleep(0.1)
                 return HttpResponse("Hello World!")
             except asyncio.CancelledError:
                 # Set the flag.
@@ -524,6 +526,7 @@ class ASGITest(SimpleTestCase):
         self.assertNotEqual(handler_call["thread"], threading.current_thread())
         # The signal sender is the handler class.
         self.assertEqual(handler_call["kwargs"], {"sender": TestASGIHandler})
+        view_started.clear()
 
         # Request cycle with a disconnect before the view can respond.
         application = TestASGIHandler()
@@ -531,7 +534,7 @@ class ASGITest(SimpleTestCase):
         communicator = ApplicationCommunicator(application, scope)
         await communicator.send_input({"type": "http.request"})
         # Let the view actually start.
-        await asyncio.sleep(0.1)
+        await view_started.wait()
         # Disconnect the client.
         await communicator.send_input({"type": "http.disconnect"})
         # The handler should not send a response.