gh-160295: Fix race in test_forkserver_sigkill (GH-160296)

After a SIGKILL, the forkserver can no longer report the first child's
exit status, so proc.join() returns with exit code 255 while the child
may still be running. Wait for the event instead of checking it right
after join().

Lib/test/_test_multiprocessing.py

@@ -994,7 +994,9 @@ class _TestProcess(BaseTestCase):
         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):