Following changes are included:
1. Move test file into third_party/.../muxer/...
2. Rewrite tests for stts box and stsz box to use dump file infra instead of Mp4Slicer.
3. Remove existing test cases related to stts box which are kind of duplicate as it tries to cover the scenarios which are already covered by "DurationsVuForStts" related test cases.
PiperOrigin-RevId: 510977119
