message: doc: Add test message to protocol docs
diff: - [[file:status.md#ok][Ok]]
  - [[file:status.md#error][Error]]
  - [[file:status.md#ping][Ping]]
  + - [[file:status.md#test][Test]]
  - [[file:status.md#requestlog][RequestLog]]
  - [[file:status.md#log][Log]]
  - [[file:identification.md#identification-messages][Identification Messages]]
  @@ -492,7 +493,7 @@ while a client communication thread continues to work.
  *Expected Response:*
  -- Ok message with matching Id on successful logging request.
  +- Ok message with matching Id on successful ping.
  - Error message on value or message error.
  *Flow Diagram:*
  @@ -516,6 +517,44 @@ msc {
  }
  ]
  #+END_SRC
  +*** Test
  +*Description:* The Test message is used for development and testing
  +purposes. Sending a Test message with a string to the server will
  +cause the server to return a Test message. If the string is "Error",
  +the server will return an error message instead.
  +
  +*Fields:*
  +
  +- /Id/ (unsigned int): Message Id
  +- /TestString/ (string): String to echo back from server.
  +
  +*Expected Response:*
  +
  +- Test message with matching Id and TestString on successful request.
  +- Error message on value or message error, or TestString being 'Error'.
  +
  +*Flow Diagram:*
  +
  +#+BEGIN_SRC mscgen :file test_diagram.svg :filetype svg
  +msc {
  + Client,Server;
  + Client -> Server [ label = "Test, TestString=Moo, Id=5" ];
  + Server -> Client [ label = "Test, TestString=Moo, Id=5" ];
  +}
  +#+END_SRC
  +
  +*Serialization Example:*
  +
  +#+BEGIN_SRC json
  + [
  + {
  + "Test": {
  + "Id": 5,
  + "TestString": "Moo"
  + }
  + }
  + ]
  +#+END_SRC
  *** RequestLog
  *Description:* Requests that the server send all internal log messages
  to the client. Useful for debugging.
  