How can I test subscriptions?

If you plan to test the auto-charging subscription then it will be best to set up Shopify Payments in your store in test mode and place a test subscription order in your shop. Here is how to turn on the test mode: https://help.shopify.com/en/manual/payments/shopify-payments/testing-shopify-payments#enable-test-mode-for-shopify-payments

If you want to test the recurring invoice subscriptions, then you can just manually create a recurring invoice subscription in Seal Subscriptions -> Subscriptions -> Add subscription manually.