How do I test Online Contribution Receipts?
I would recommend you only TEST LIVE; Just delete the contribution that you have created afterwards; a $1 LIVE transaction that you can follow all the way though into your Payment Processor - will guarantee you that things are working as expected. 

Added: You'll have tokens for total_amount etc, right? This is how you test it. You can also plug in TEST credentials for your payment Processor in LIVE payment processor config. Then you can test LIVE CiviCRM pathways with fake VISA cards.