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>
OK I wasn't sure that `self.stuff` is initialized during `setup`. I'll change that.