[FIX] purchase_mrp: fixing test test_valuation_with_backorder

this commit https://github.com/odoo/odoo/pull/158788/commits/07e2fca9e39e9c32182f10bae6ba814a4241b6bf
uses `qty_done` instead of `quantity` in test
`test_valuation_with_backorder`

closes odoo/odoo#159248

Signed-off-by: Xavier Dollé (xdo) <xdo@odoo.com>

addons/purchase_mrp/tests/test_purchase_mrp_flow.py

@@ -971,8 +971,8 @@ class TestPurchaseMrpFlow(AccountTestInvoicingCommon):
         po.button_confirm()
 
         receipt = po.picking_ids
-        receipt.move_line_ids[0].qty_done = 4
-        receipt.move_line_ids[1].qty_done = 2
+        receipt.move_line_ids[0].quantity = 4
+        receipt.move_line_ids[1].quantity = 2
         action = receipt.button_validate()
         wizard = Form(self.env[action['res_model']].with_context(action['context'])).save()
         wizard.process()