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 and in the case refund is required, you will know how to go about processing it.
For a test order, you can set a minimum amount to reduce and minimize costs, in case there is an additional cost incurred.