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>
Just a reminder:  You can bundle static data along with the test, then just copy it into ``srcdir``.  Sometimes this is easier if the steps to create the may change but the data wouldn't need to.  What you have here is fine though, it's simple enough.