[3.15] gh-160295: Fix race in test_forkserver_sigkill (GH-160296) (#160508)

Co-authored-by: Davi de Jesus <davidejesus.log@proton.me>

Author: miss-islington

Lib/test/_test_multiprocessing.py

@@ -993,7 +993,9 @@ def check_forkserver_death(self, signum):
         self.assertEqual(proc2.exitcode, 0)
 
         proc.join()
-        self.assertTrue(evt.is_set())
+        # After SIGKILL, join() can return (exitcode 255) as soon as the
+        # status pipe is closed, before the orphaned child has run.
+        self.assertTrue(evt.wait(support.SHORT_TIMEOUT))
         self.assertIn(proc.exitcode, (0, 255))
 
     def test_forkserver_sigint(self):