Use new API in test mode (http://payment-beta.klarna.com:80)
Change add_invoice to work with new Klarna API (params changed? https://developers.klarna.com/de/more/deprecated-services/functions#addTransaction)
Use Klarna Testuser for Tests (https://developers.klarna.com/de/testing/invoice-and-account)
Fix some tests using response from add_invoice as invoice_no but it was an array (maybe not do this in side test...)