[3.11] gh-116682: stdout may be empty in test_cancel_futures_wait_false (GH-116683) (#116693)

If the `shutdown()` call happens before the worker thread starts executing
the task, then nothing will be printed to stdout.
(cherry picked from commit 7d1abe9)

Co-authored-by: Sam Gross <colesbury@gmail.com>

Author: miss-islington

Lib/test/test_concurrent_futures/test_shutdown.py

@@ -247,7 +247,9 @@ def test_cancel_futures_wait_false(self):
         # Errors in atexit hooks don't change the process exit code, check
         # stderr manually.
         self.assertFalse(err)
-        self.assertEqual(out.strip(), b"apple")
+        # gh-116682: stdout may be empty if shutdown happens before task
+        # starts executing.
+        self.assertIn(out.strip(), [b"apple", b""])
 
 
 class ProcessPoolShutdownTest(ExecutorShutdownTest):