Use a better sync for test_exception_swallowed_while_serving test.

* bzrlib/tests/test_test_server.py:
(TestTCPServerInAThread.test_exception_swallowed_while_serving.FailingWhileServingConnectionHandler):
We need to make sure the exception has been recorded which
requires using the thread event.

* bzrlib/tests/test_server.py:
(TestingTCPServerMixin.serve): Make sure we clear the 'stopped'
event.
(TestingTCPServerMixin.shutdown_socket): Can also be used for the
server socket.
(TestingTCPServerInAThread.stop_server): Swallow ignored
exceptions when joining the server thread.