[AppConfig] Close async clients (Azure#27287)

Author: Yalin Li

sdk/appconfiguration/azure-appconfiguration/tests/asynctestcase.py

@@ -47,5 +47,6 @@ async def tear_down(self):
             config_settings = self.client.list_configuration_settings()
             async for config_setting in config_settings:
                 await self.client.delete_configuration_setting(key=config_setting.key, label=config_setting.label)
+            await self.client.close()
         else:
             raise ValueError("Client is None!")