Sorry to hear about your injury. Can I ask is it the only incident in the dojo or do they happen often? Do they happen only when you uke for certain students or is it a common thing? Aikido is considered a safe practice but sometimes accidents do happen, no one is perfect (even if they wear a hakama).
Try to speak to your instructor personally, when they have time to listen to your concerns. It may have been that before the tests the instructor was a bit nervous, concerned about other things, too many people on the mats, that for some reason he was not listening when you told him. And while it is not really acceptable, everyone is only a human.
Come on guys, have faith in your fellow Aikidokas! Everyone is learning, including our instructors. If you let your sensei know how you feel, he may learn something from it as well. We all learn and sometimes we may have to pay for this education with a bit of scalp or another injury (been there). And while the practice must be made as safe as possible, accidents are accidents.
Anonymous, I hope this situation will not put you off Aikido. I live in an area with only one dojo and I think understand how you feel.
Hope you recover soon.