If you'd like to delete a test order there are two steps.
Step 1: Delete the order
On your Shopify admin dashboard, delete the order.
Step 2: Cancel the subscription
In Ongoing, click on Subscriptions.
Click on the test subscription you'd like to cancel
Under Subscription status, click Cancel, and save status
This will ensure that you have deleted not just the order connected to the subscription but also the ongoing subscription itself.
Read instructions on how to cancel a subscription (complete with pictures).