Clarify some iable/method names in the test servers.
* bzrlib/tests/stub_sftp.py: (SocketListener.__init__): '_connection_callback is more explicit than '_callback'. (SFTPServer._run_server_entry): Mention that SocketDelay is probably ineffective for the connections themselves. (SFTPServer._run_server): 'ready' is more explicit than 'event'.
* bzrlib/tests/http_server.py: (HttpServer.run_server): 'run_server' is more explicit than http_start.