If you manage a civiCRM instance and use iATS for your payment processor and would like to test a contribution, here's what I had to do.
Log into your 1stPay Transaction Center (https://secure.1stpaygateway.net/secure/transcenter/index.aspx) and go to the Security Settings/Gateway Options page (https://secure.1stpaygateway.net/secure/transcenter/security/security.aspx?target=security_gatewaykey)
Enable test mode. This will create a sandbox for you. This is a test instance of the gateway with its own transaction searches etc. The menu in the upper right hand corner will let you know which instance you are in and if you are in test mode, a large RED banner at the top will say TEST MODE ENABLED.