subtests.docker_cli: Add new test load_similar

Fixes: https://github.com/autotest/autotest-docker/issues/346

This test creates image with file /testfile. Than it saves this image
and modifies the content of the /testfile. Then it loads the original
image and verifies the file contain correct value. Without any changes
it loads the corrupted version and verifies that the value was modified.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
These three should probably go into ``stuff``.  You can still set them to ``None`` because ``initialize()`` runs before ``setup()``.  That also lets you use ``stuff.get()`` in case it needs to default on something.