Sales orders in done state cannot be deleted in the regular OpenERP way. Truncating a database is generally a bad idea. Some complications can occur because of lost connections and you will have no idea where it might crash.

It is nearly always better to just start over with a clean database. Perhaps make some backups during the testing so you can always restore to a step in the process. 

If you really still insist on truncating the database, then perhaps look up the sale_order or sale_order_line table and set all the relations to the pickings to false.