I know my local skating clubs all have written policies on withdrawals from competitions, but none of the clubs I've tested at seem to have a written policy on withdrawing from a test session.
IMO, it would be really hard to have an open policy on this. You guys are volunteers, and it's hard enough to schedule the test session and deal with trying to get judges and etc., never mind having to handle reimbursing skaters who simply choose not to show up. In addition, you've already bought the ice, obtained the judges for a certain number and level of skaters. So having the ability to have skaters withdraw once all that is done can really impact your club's finances and perhaps reputation. If you did this, you'd probably need a written policy, as you noted, and to have a hard deadline date after which refunds can't be made or, if you choose not to allow refunds, after which the skater's test can't be moved to the next test session.