[FIX] sale_coupon: fiscal position for promotion product line

create a SO;
Add a fiscal position;
Add a promotion of fixed amount kind.
Before this commit, the taxes of the promotion are the one of the
promotion product and not the one of the fiscal position.

Now, the taxes will be computed taken into account the fiscal position.

opw-2467966

closes odoo/odoo#67013

Signed-off-by: Jorge Pinna Puissant (jpp) <jpp@odoo.com>