Start implementing the threading variants.

* bzrlib/tests/test_test_server.py:
(TestTestingServerInAThread, TestTestingThreadingServerInAThread):
Test the threading variants.

* bzrlib/tests/test_server.py:
(TestingTCPServerMixin.__init__): Keep track of the sibling class
to better share the code.
(TestingTCPServerMixin.server_bind): Call the sibling class.
(TestingThreadingTCPServer, TestingThreadingTCPServerInAThread):
The threading versions.