Lightstreamer Client Library is based on the TLCP application protocol, that uses HTTP and Websocket as transport protocols. Since TLCP is stateful, it is more difficult to mock a server based on it than to mock a server based on stateless paradigms such as HTTP/REST.
At the moment there is no out-of-the-box test framework which provides the required capabilities. So our suggestion is simply to wrap the Lightstreamer Client is a component and then to mock the component in your tests.