Create a sanity check

[Ceylo]

At the moment there is no basic working test to check that playback is ok. Because of this, for 2 weeks we didn't see that the subtitles weren't working anymore (invalid position).

Write basic tests to check that:

• [ ] opening on some basic media files work
• [ ] gathered properties (streams, duration, image size, etc) are correct
• [ ] the first image isn't black and is completely in the window
• [ ] the first subtitle isn't black and is within the window bounds
• [ ] the amount of different frames sent to the window exactly matches the count of video frames in the media