Not sure. Static or singleton-pattern LevelManager with a LoadLevel(string sceneName) function that handles the connection process? You basically just call TNManager.Connect(serverAddr, serverPort) and then TNManager.JoinChannel(-1, sceneName, ...) when TNManager.onConnect fires and you've got your networked scene ready to go.
Though for testing purposes you don't really need a connection, do you? I guess it depends on what you're testing, but I think RFCs and RCCs work even without a connection. Actually, I think everything may work without a connection in TNet3, even saving player data and whatnot.