[IMP] sale_stock: test propagation of incoterm on down-payment

Ensure that, when creating a down-payment invoice for a SO with an incoterm, this invoice should have the same incoterm as the SO.

Steps:
1. Create a SO with an incoterm
2. Create a down-payment invoice for this SO
3. The new invoice should have the same incoterm as the SO

closes odoo/odoo#122538

Opw: 3284556
X-original-commit: 87542160
Signed-off-by: Adrien Widart (awt) <awt@odoo.com>
Signed-off-by: malv-odoo <malv@odoo.com>