[18.0] [MIG] pos_session_pay_invoice

[Tisho99]

Standard migration

• The pos order is now created in draft state because we can no longer create pos payments in posted pos orders. The 'invoiced' state is assigned after that
• The refund of the setUpClass is now generated from a new invoice because in v18 the original and refund moves were being automatically conciliated

Depends of OCA/account-payment#820

T-9200