[FIX] sale_coupon: fix step like programs discount

Create four promotion programs:

if the order > 1500 than 10% discount
if the order > 1750 than 15% discount
if the order > 2000 than 20% discount
if the order > 2500 than 25% discount

Take a product with a price of $300 and add 5 to cart > the 10% discount
is correctly applied.
Add 1 more product (6) > it should now qualify for 15% discount, but it
stays at 10%
Add 1 more product (7) > it (correctly) gets the 20% discount
The 25% discount does not get applied until 11 of the product is in the
cart, even though it should qualify at 9
If you then decrease the quantity, the right discount will
sometimes display.

This occur because when the order amount change and the new total is
used to match the right promo the previously applied discount is not
removed from the amount.
This occur as side effect of
1d59785 in which the amount has to be
kept in order to avoid discount line removal on cart update.

opw-2285656

closes odoo/odoo#54400

X-original-commit: 28c099e6d5b02518b6b5a0be66b2b74a57f0c936
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>