Add test code which will crawl a fake internal list of checkpoints and
generate the list of bitmaps for merging to gather the final bitmap for
the backup.

The initial tests cover the basic case of all bitmaps being present in
the top layer of the backing chain.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>