[FIX] l10n_account_edi_ubl_cii_tests: adapt test with prepaid amount

Adapt the tests after commit
https://github.com/odoo/odoo/commit/47905d21ab65f985291d78cb7ecb78f2f7cdcf67


A down payment is no longer created when a prepaid amount is detected in
a UBL/CII attachment.

closes odoo/odoo#120653

Signed-off-by: Laurent Smet <las@odoo.com>