test_librbd_fsx: also flatten as part of randomize_parent_overlap

With randomize_parent_overlap fsx will randomly truncate base images
after they have been cloned from.  This throws flatten into the mix:
base image will be flattened with 2/16 chance (equal to the chance of
leaving the image intact).

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>