pythongh-127258: Fix asyncio test_staggered_race_with_eager_tasks()

Replace the sleep with a task which is blocked forever.

Author: vstinner

Lib/test/test_asyncio/test_eager_task_factory.py

@@ -220,10 +220,14 @@ async def fail():
             await asyncio.sleep(0)
             raise ValueError("no good")
 
+        async def blocked():
+            fut = asyncio.Future()
+            await fut
+
         async def run():
             winner, index, excs = await asyncio.staggered.staggered_race(
                 [
-                    lambda: asyncio.sleep(2, result="sleep2"),
+                    lambda: blocked(),
                     lambda: asyncio.sleep(1, result="sleep1"),
                     lambda: fail()
                 ],