Add a test script for Mousepad

I started to write a test script for Mousepad. For the moment it tests:

• Running Mousepad with the various command line options and quitting as soon as the window appears on the screen
• Setting change via gsettings when Mousepad is running (which contains about the test of the application actions)
• Window actions activation via gdbus

Maybe I'll expand it later by simulating user inputs with xdotool or such, or to test more specifically this or that action by other means.

I think this could be useful to ensure a minimum threshold of non-regression, especially when making large-impact refactoring code changes.