test_filestore_idempotent: use obj name from source coll add

Fixes: #5240
Reviewed-by: David Zafman <david.zafman@inktank.com>
Signed-off-by: Samuel Just <sam.just@inktank.com>
rados: --num-objects will now cause bench to stop after that many objects

Reviewed-by: David Zafman <david.zafman@inktank.com>
Signed-off-by: Samuel Just <sam.just@inktank.com>