testing/rnote: Added rnote-cli subpackage

I've enabled the rnote command line interface, which can be found in the rnote-cli subpackage.

$ rnote_cli
rnote_cli  Copyright (C) 2023  The Rnote Authors

 This program is free software; you can redistribute it and/or modify it under the terms of the GPL v3 or (at your option) any later version

Usage: rnote_cli <COMMAND>

Commands:
  test    Tests if the specified files can be opened and are valid rnote files
  import  Imports the specified input file and saves it as a rnote save file.
               Currently only `.xopp` files can be imported
  export  Exports the Rnote file(s) and saves it in the desired format.
               When using --output-file, only one input file can be given.
               The export format is recognized from the file extension of the output file.
               When using --output-format, the same file name is used with the extension changed.
               --output-file and --output-format are mutually exclusive but one of them is required.
               Currently `.svg`, `.xopp` and `.pdf` are supported.
               Usages: 
               rnote-cli export --output-file [filename.(svg|xopp|pdf)] [1 file]
               rnote-cli export --output-format [svg|xopp|pdf] [list of files]
  help    Print this message or the help of the given subcommand(s)

Options:
  -h, --help     Print help
  -V, --version  Print version