[FIX] update module unit test to use action_assign method before every picking transfer operation

Created by: zaoral

module: stock_picking_invoice_validation

A quick fix was made here https://github.com/Vauxoo/addons-vauxoo/pull/783 to make one of our clients instance that depends of this module to work. So the unit test pass when trying to transfer a picking.

The quick fix was add a force_assign before every picking transfer. This PR want to remove this temporal fix and make a real fix.

The real fix refers to update product demo data used in modules unit tests to have enough availability for test, and replace the force_assign with action_assign