thrashosds.py: fix line length

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
thrashosds: add test_backfill_full

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>