Isolated synchronizer + add new tests

Details:
 - Added dummy driver to avoid relying on melted-node-driver
 - Added tests:
    - Responses to play_upstream event
       - with no clips (emits sched_upstream)
       - after dataupdated event (emits sched_upstream)
       - after datasend event (syncroScheduledClips() & emits sync_downstream)
       - playing with no new data (emits sync_downstream)
       - playing but different playlists (syncroScheduledClips() & emits sync_downstream)
       - playing with less clips than needed (emits sched_upstream)
Fix bug raised by the test "play_upstream with different playlists"