Add test on handling PlayerMediator events on its creation.
Add constant of path to test mp3 file.
Use mp3 test file constant in existen test.
Add test which adds two track to PlayerMediator.
Return bool instead of GstStateChangeReturn in CommonPlayer.
Fix test with running Play method on CommonPlayer.
Add file with test utils functions.

Add function which gets media file by url.
Use unified util function for getting test media file.
Add test which pauses track and resume it.
Rename test setup_playlist_and_play -> setup_playlist_and_play.
Add test which stops playing of track and resume it.
Catch unhandled exception on playing track.
Add test which run play commands on empty playlist.
Do not do loop of playlist by default.
Add tests which changes tracks in playlist.
Use _TRUE assert instead of "_EQ(true, ".
Merge branch 'rework-returning-state-of-player-control-methods' into add-player-mediator-tests
Fix typo in documentation of ObserverMediator.
Make pause on Play() if Player is playing.
Handle change of Player state.