Magento 2 emulating (debugging) Thank you (success) page

File vendor/magento/module-checkout/Controller/Onepage/Success.php

Should look like:

public function execute()
    {
        $session = $this->getOnepage()->getCheckout();
        $session
            ->setLastOrderId(OLD_ID)
            ->setLastRealOrderId('OLD INCREMENT ID')
            ->setLastOrderStatus('Pending');