gh-110395: test: assert after the child dies. (#111816)

based on review from Victor Stinner.  I already made this edit in the 3.12 backport PR.

Lib/test/test_kqueue.py

@@ -271,8 +271,8 @@ def test_fork(self):
             finally:
                 os._exit(0)
         else:
-            self.assertFalse(kqueue.closed)
             support.wait_process(pid, exitcode=0)
+            self.assertFalse(kqueue.closed)  # child done, we're still open.
 
 
 if __name__ == "__main__":