Make sure that a forked child process exits even if it raises an exception.

Lib/test/test_mailbox.py

@@ -735,9 +735,11 @@ class _TestMboxMMDF(TestMailbox):
         pid = os.fork()
         if pid == 0:
             # In the child, lock the mailbox.
+            try:
                 self._box.lock()
                 time.sleep(2)
                 self._box.unlock()
+            finally:
                 os._exit(0)
 
         # In the parent, sleep a bit to give the child time to acquire