[test] Add checks for created images
    
    The performed checks:
     * If format is ISO, the file must have correct magic string
     * If it's bootable, there must be MBR or GPT
    
    When a check fails on any failable deliverable, it will be logged and
    the file removed from metadata (it will still remain on the disk). This
    required a change to write the images.json file later (after test
    phase).
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>