There is no test mode in Systeme (at the moment), so the only way to ensure that your check-out page working as intended is to perform a live test. You can then process a refund to understand how the refund works.
So a live test transaction with the refund prepares you for live customers’ orders. The test will ensure payment can go through; if a refund is required, you will know how to process it.
For a test order, you can set a minimum amount to reduce and minimize costs in case an additional cost is incurred.