Return values instead of changed result when testing payment term

Since 0db956eb2505 the returned value of on_change_with is set on the field.

Closes #12775
(grafted from aea01e3d595a526f7a093b0f7cef59ad215cab2a)

--HG--
branch : 7.0

modules/account_invoice/payment_term.py

@@ -347,8 +347,7 @@ class TestPaymentTermView(ModelView):
                         date=date,
                         amount=amount,
                         currency=self.currency))
-        self.result = result
-        return self._changed_values.get('result', [])
+        return result
 
 
 class TestPaymentTermViewResult(ModelView):