Test a webhook

Sends sample notifications to test if the webhook is set up correctly. We send sample notifications for maximum 20 of the merchant accounts that the webhook is configured for. If the webhook is configured for more than 20 merchant accounts, use the `merchantIds` array to specify a subset of the merchant accounts for which to send test notifications. We send four test notifications for each event code you choose. They cover success and failure scenarios for the hard-coded currencies EUR and ...