check_multiextent.sh uses three different ISOs with nearly identical content.
All three images exercise libcdio's capability to present multiple extents with
the same names as one single file. They were produced by a GNU xorriso binary
which was built for an artificially small extent size, so that the payload file
of 55 KB is split into seven extents. Normally multiple extents emerge only for
data files of 4 GiB or larger.

test/data/multi_extent_8k.iso
  is a minimally sized xorriso image without a Joliet tree, so that libcdio
  will choose to interpret the Rock Ridge information of the ISO 9660 tree.

test/data/multi_extent_8k_emul_toc_padded.iso
  is a xorriso image with the same content as multi_extent_8k.iso but larger
  size, because two default properties of a native xorriso run have not been
  disabled when it was produced.
  These properties are:
  - a second superblock which would survive when another session would be added
    and so would provide an opportunity to mount the state of the first
    session. This would also enable xorriso to show a history of all sessions
    if more than one is present.
  - traditional end padding of 300 KiB which would prevent the Linux readahead
    bug which can cause i/o errors when the ISO is presented to Linux on a CD
    that was written with write type Track-At-Once.

test/data/multi_extent_8k_joliet.iso
  is much like multi_extent_8k.iso but does not offer Rock Ridge information in
  its ISO 9660 tree. Instead it contains a Joliet tree which libcdio prefers
  over ISO 9660.

