Made test_start_task_soon_cancel_immediately() less flaky

Author: agronholm

tests/test_from_thread.py

@@ -419,17 +419,21 @@ def test_start_task_soon_cancel_immediately(
  self, anyio_backend_name: str, anyio_backend_options: dict[str, Any]
  ) -> None:
  cancelled = False
+ done_event = threading.Event()
 
  async def event_waiter() -> None:
  nonlocal cancelled
  try:
  await sleep(3)
  except get_cancelled_exc_class():
  cancelled = True
+ finally:
+ done_event.set()
 
  with start_blocking_portal(anyio_backend_name, anyio_backend_options) as portal:
  future = portal.start_task_soon(event_waiter)
  future.cancel()
+ done_event.wait(10)
 
  assert cancelled