[Help wanted] Unit testing

[h4tr3d]

Good things. But I have no time and ffmpeg-related project for this time to implement it, so any help will be welcomed.

My preferred C++ test framework is Catch2

1. Use current implementation of the https://github.com/h4tr3d/avcpp/tree/master/example/api2-samples as base for tests.
2. Create another repository that can be submoduled via master repo and will contains media data for tests. Good base for media data is Big Buck Bunny.
3. Add tests running to the CI configuration.