with paypal payments standard, there is not really a way of making test transactions.
paypal standard works like this:
* on the confirm page, the user clicks the checkout button and them is brought to the paypal site
* on the paypal site they chose their payment method, if paypal accepts that method as a valid payment method, the user is directed back to your site.
* At this point the transaction is placed in queue to be processed on a first come first served basis. this can take seconds, hors or even days for the transaction to be processed with the merchant bank.
* when the transaction is processed, the paypal system will use the notification method you have set up in your paypal account, most common is to use IPN.
* If you use IP a POST will be sent to the page yo have specified to receive the IPN POST and process the information
the IPN receiving page is a page that you set up to receive the IPN POSt and store information about the transaction in your database.
It's really only the IPN page that needs sand box testing
Information on IPN Testing can be found here:
PayPal standard is a difficult gateway to deal with, it is not a simple one to set up.
If you are looking for a free option that is easier to set up, I suggest using Paypal Express checkout.