[FIX] website_sale: fiscal position application

Have the following taxes:
- Tax A: 15%, included in price
- Tax B: 21%, included in price

Create a product P with list price 115 and tax A

Have a fiscal positions FPOS configured with:
- Detect Automatically checked
- Country: Belgium
- Map tax on product: A -> B

Open a web shop session as guest, add P to cart
Go to checkout: Price total will be 115
Fill address details with Belgium address
Go to confirmation page

Error:
Price total will be 100. In the application of fiscal position the wrong
unit price is used

opw-2973879

closes odoo/odoo#101364

Signed-off-by: William Braeckman (wbr) <wbr@odoo.com>