test/py: rewrite common tools for SquashFS tests

Remove the previous OOP approach, which was confusing and incomplete.
Add more test cases by making SquashFS images with various options,
concerning file fragmentation and its compression. Add comments to
properly document the code.

Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org> [on sandbox]
Signed-off-by: Joao Marcos Costa <jmcosta944@gmail.com>