Click VIEW under Manage Test Users and it will show your test subscriptions. To be able to do test purchases, Add New Test User. Also click EDIT to set the channels you're able to test. Otherwise, your purchase will probably go through as a real transaction and you're going to be stuck with it until you cancel or it expires in 30 days.
Doing Test purchases of your own product codes probably won't work from a side-loaded channel. You'll want to upload your latest channel package like you're ready to submit an update but instead of submitting it for publication, click on the Access Code which will add the channel to your Roku device for you to preview and test, which as far as I know is exactly the same but much quicker than making a separate private channel to do testing. The channel will be added to the bottom of your Roku grid with the newer version so you won't mistake it for your current live version of the channel.
One other thing that will save you a lot of grief... if you just added a new product code, Roku used to show "Approved" some short time afterwards when a new product was fully registered in their system. Now Roku immediately says it's "Approved" even though it's not fully ready for sale. You'll know it's not ready when you try to do a test purchase and your channel kicks back an error dialog with no explanation what went wrong. It basically means the product isn't available yet. For me it takes 15-30 minutes for a new product to be fully ready.