import_export.sh: sparse import export

import_export.sh: sparse import export

Add tests for:
   - sparse import makes expected sparse images
   - sparse export makes expected sparse files
   - sparse import from stdin also creates sparse images
   - import from partially-sparse file leads to partially-sparse image
   - import from stdin with zeros leads to sparse
   - export from zeros-image to file leads to sparse file

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>