Test mode allows you to upload payslips without the Client or Employees being notified. Any emails sent as part of the payroll upload that would go to Employees of the Client are all sent to you instead so that you can see what is happening. There is a separate setting on the final page of the payroll upload as to whether to send out emails at all.
You can change a client's Test Mode status from the client settings page. You can access this from the Agency Dashboard by clicking CLIENTS and then MANAGE next to the required client. You will then be taken to the client settings page. From here, click EDIT DETAILS and you will see the option to change the setting on the next page.