account-closing [16.0][FIX] account_cutoff_accrual_sale : Fixes for SO invoiced on order + [ADD] support for down payment

I added pleeeeenty of tests !

Covering:

normal service on order : should be invoiced when SO is confirmed
shipping service on order : should be invoiced when first stock delivery occurs (that's a specificity of delivery module https://github.com/odoo/odoo/blob/16.0/addons/delivery/models/sale_order.py#L155)
stock product on order : should be invoiced when delivered (invoicing policy doesn't matter here, accrual is based on stock valuation change, i.e. the effective stock delivery)

cc @rousseldenis @lmignon @sbidoul

Also fixed account_cutoff_accrual_order_base that was updating stock in the test while not depending on stock module. Code moved to modules depending on this one and stock

Edit: I also added support for downpayment