There are some resource leaks:

* When error occurred in setUp() function, server thread is not stopped.  It leaked threads and sockets for the server.
* When error occurred in server thread's run() method, asyncore.close_all() is not called.

These leaks makes dangling threads.  It caused hang in test_poplib shutdown process.

Additionally, the error is not printed in these cases.  It makes harder to fix real problem.