test: add default track selection testing

The amount of options we have that are related track selection is insane
and touching any of this stuff is scary. Add some track selection
testing to the CI to hopefully make it slightly less scary to touch any
of that. Since trying to work in media files from some outside source
would be a pain and need to live in some repo somewhere, it's nicer to
just generate dummy files on the fly with ffmpeg during the ci runs and
use that. Obviously, the actual tests themselves could be even more
thorough (external tracks were not even considered), but this is more
than a good enough start for now.