test_ws_server: Add basic tests for WebSocket[Server/Client]Receiver

This patch adds nominal tests for the WebSocketServerReceiver and the
WebSocketClientReceiver.

In the case of the WebSocketServerReceiver, these tests don't cover much just
yet, as the handling for messages is going to get some heavy work sooner
rather than later. However, it provides the location for where those tests
will eventually go.

In the same fashion, since a WebSocketClientReceiver will be heavily dependent
on the usage - that is, if the client is a MUD server instance or a command
line client - the tests don't have to cover much, as the class does not do
much.