Fix test_worker_waits_for_scheduler (#6155)

Remove asyncio.wait_for from test.
It seems to be malfunctioning very rarely.
We replace it with a less sophisticated task + sleep + cancel.

Fixes #6153

distributed/tests/test_worker.py

@@ -325,12 +325,15 @@ async def test_worker_port_range(s):
 @gen_test(timeout=60)
 async def test_worker_waits_for_scheduler():
     w = Worker("127.0.0.1:8724")
-    try:
-        await asyncio.wait_for(w, 3)
-    except TimeoutError:
-        pass
-    else:
-        assert False
+
+    async def f():
+        await w
+
+    task = asyncio.create_task(f())
+    await asyncio.sleep(3)
+    assert not task.done()
+    task.cancel()
+
     assert w.status not in (Status.closed, Status.running, Status.paused)
     await w.close(timeout=0.1)