Port the iuk test suite from squashfs-tools to squashfs-tools-ng

refs #18577 (closed)

Closes #18653 (closed)

Note to reviewer:

• to run the test suite: https://tails.boum.org/contribute/release_process/tails-iuk/
• part of the test suite is broken, like it is on stable, due to #18661 (closed).