test/py: rewrite sqfsls command test suite

Add more details to test cases by comparing each expected line with the
command's output. Add new test cases:
- sqfsls at an empty directory
- sqfsls at a sub-directory

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>