Add unit tests for distance-based video sampling, GPMF parser, and exiftool video reading

- sample_video: Add 20 tests for _within_track_time_range_buffered,
  _sample_video_stream_by_distance, rerun behavior, and end-to-end
  distance-based sampling with mocked ffmpeg/geotag/MP4 parsing
- test_gpmf_parser: Expand GPMF parser test coverage; remove tests for
  extract_camera_model (removed in dead-code-cleanup)
- test_exiftool_read_video: Add new test file for exiftool video reading

Author: caglarpir