[FIX] stock_landed_costs: flaky tests

We chose the vendors by searching random res.partner but sometimes
partners of another company are returned, making the _check_company
mechanism raise an error.

closes odoo/odoo#44815

X-original-commit: 1cfcf21efa888720f9c34b337532251db6fd0089
Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>