Tests various use cases when cloning MessagePorts.

Should be a series of SUCCESS messages, followed with DONE.

SUCCESS: Posting port to itself: Error: DATA_CLONE_ERR: DOM Exception 25
SUCCESS: Posting entangled port: Error: DATA_CLONE_ERR: DOM Exception 25
SUCCESS: Posting cloned port.
SUCCESS: Posted messages to cloned port.
SUCCESS: Cloned both endpoints of a channel.
SUCCESS: Got closed port with event message closed
DONE