test/py: rewrite sqfsload command test suite

The previous strategy to know if a file was correctly loaded was to
check for how many bytes were read and compare it against the file's
original size. Since this is not a good solution, replace it by
comparing the checksum of the loaded bytes against the original file's
checksum. Add more test cases: files at a sub-directory and non-existent
file.

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>