add async tests

Author: srtaalej

tests/slack_sdk_async/web/test_async_chat_stream.py

@@ -244,6 +244,23 @@ async def test_streams_a_chunk_message(self):
                 '[{"text": "\\n", "type": "markdown_text"}, {"text": ":space_invader:", "type": "markdown_text"}]',
             )
 
+    @async_test
+    async def test_ts_property_is_none_before_flush_and_set_after(self):
+        streamer = await self.client.chat_stream(
+            buffer_size=5,
+            channel="C0123456789",
+            thread_ts="123.000",
+            recipient_team_id="T0123456789",
+            recipient_user_id="U0123456789",
+        )
+        self.assertIsNone(streamer.ts)
+
+        await streamer.append(markdown_text="hello!")
+        self.assertEqual(streamer.ts, "123.123")
+
+        await streamer.stop()
+        self.assertEqual(streamer.ts, "123.123")
+
     @async_test
     async def test_streams_errors_when_appending_to_an_unstarted_stream(self):
         streamer = await self.client.chat_stream(