Connection: now it refuses to send/receive frames after it sends/receives a close frame. (RFC6455 1.4, 5.5.1)
done_testing is not necessary in subtests.
testlib::Connection: add create_connection_and_handle() method for refactoring.
Connection: now it aborts the connection when it receives a too huge message. The size limit is imposed by Protocol::WebSocket::Frame (RFC6455 10.4)