Fix the cancelling of subscriptions

src/python_testing/TC_DEM_2_10.py

@@ -150,7 +150,7 @@ async def test_TC_DEM_2_10(self):
         asserts.assert_less_equal(count, 10, "More than 10 reports received")
 
         self.step("9")
-        sub_handler.cancel()
+        await sub_handler.cancel()
 
         self.step("10")
         await self.send_test_event_trigger_forecast_adjustment_clear()

src/python_testing/matter_testing_support.py

@@ -422,10 +422,15 @@ async def start(self, dev_ctrl, node_id: int, endpoint: int, fabric_filtered: bo
         self._subscription.SetAttributeUpdateCallback(self.__call__)
         return self._subscription
 
-    def cancel(self):
+    async def cancel(self):
         """This cancels a subscription."""
-        self._subscription.Shutdown()
-        self._subscription = None
+        # Wait for the asyncio.CancelledError to be called before returning
+        try:
+            self._subscription.Shutdown()
+            await asyncio.sleep(5)
+        except asyncio.CancelledError:
+            pass
+
 
     def __call__(self, path: TypedAttributePath, transaction: SubscriptionTransaction):
         """This is the subscription callback when an attribute report is received.