Update tests to use a more current image dict
The image dict used in test_image is pretty ancient, and was
actually failing due to fedfind adjusting the image dict for
https://pagure.io/pungi/issue/417 . Let's just update the test
to use a much newer image dict (from the final F29 nightly).
Signed-off-by: Adam Williamson <awilliam@redhat.com>