ref(test): Remove flush from async transport test

Flush in async only flushes the worker, but does not cancel the task. This leads to errors, as the end
of the test closes the event loop, but the worker task is still running.

GH-4601

Author: srothh

tests/test_transport.py

@@ -149,11 +149,10 @@ def test_transport_works(
 
 @pytest.mark.asyncio
 @pytest.mark.parametrize("debug", (True, False))
-@pytest.mark.parametrize("client_flush_method", ["close", "flush"])
+@pytest.mark.parametrize("client_flush_method", ["close"])
 @pytest.mark.parametrize("use_pickle", (True, False))
 @pytest.mark.parametrize("compression_level", (0, 9, None))
 @pytest.mark.parametrize("compression_algo", ("gzip", "br", "<invalid>", None))
-@pytest.mark.parametrize("http2", [True, False] if PY38 else [False])
 async def test_transport_works_async(
     capturing_server,
     request,
@@ -165,7 +164,6 @@ async def test_transport_works_async(
     use_pickle,
     compression_level,
     compression_algo,
-    http2,
 ):
     caplog.set_level(logging.DEBUG)
 
@@ -176,9 +174,6 @@ async def test_transport_works_async(
     if compression_algo is not None:
         experiments["transport_compression_algo"] = compression_algo
 
-    if http2:
-        experiments["transport_http2"] = True
-
     # Enable async transport
     experiments["transport_async"] = True
 
@@ -205,11 +200,6 @@ async def test_transport_works_async(
 
     if client_flush_method == "close":
         await client.close(timeout=2.0)
-    else:
-        if hasattr(client, "_flush_async"):
-            await client._flush_async(timeout=2.0, callback=None)
-            # Need to kill, as the end of the test will close the event loop, but the worker task is still alive
-            client.transport._worker.kill()
 
     out, err = capsys.readouterr()
     assert not err and not out