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@SGaist Thank you for your reponse. I will check both QtTestModule and Squish.
Some time ago Qt published webinar concerning the topic, which helped me a lot.
I am adding a link for future readers: https://resources.qt.io/videos/complying-with-the-requirements-of-the-gpl-lgpl-v3-license-on-demand-webinar
How are you sending the command to mplayer ?