Add asyncio fixture to test_async_receiver

This ensures that this test executes and passes
with pytest-8.4+

pytest now throws errors for such functions [1]
which were skipped in older versions

Author: kraj

tests/test_signals.py

@@ -257,6 +257,7 @@ def received(sender: t.Any) -> None:
     assert [id(fn) for fn in sig.receivers.values()] == [fn_id]
 
 
+@pytest.mark.asyncio
 async def test_async_receiver() -> None:
     sentinel = []