I was just thinking that to test something like this, you'd have to join a group, trigger the send, but then wait 24 hours to make sure triggering the send again doesn't send anything after 24 hours.  If the condition instead takes into account the last time the reminder was sent or tracks which memberships have changed without a reminder having been sent, then you can just trigger the send twice and immediately see that the first trigger sends a reminder and the second doesn't.