Fix test

Author: crusaderky

distributed/tests/test_scheduler.py

@@ -624,7 +624,7 @@ def func(first, second):
         secede()
         second.wait()
 
-    fs = c.map(func, [first] * 5, [second] * 5)
+    fs = c.map(func, [first] * 5, [second] * 5, key=[f"x{i}" for i in range(5)])
     await async_poll_for(lambda: a.state.executing, timeout=5)
 
     await first.set()