I've undoubtedly had less experience in seeing and being tested than you, as I do not find the same boredom. However I can relate some experiences. I think it very much depends on the grade your testing (I presume your talking about gradings). It always seems more fun when 2nd dans or higher are being graded.
- get the best karateka (or at least a good one) to do the karate style attacks. During a grading a dan grade was thrown across the room from a Mae Geri (front kick) coming from a lower grade Aikidoka.
- always make sure there is maximum pressure on the student. This can be achieved by:
1. inviting the highest grade sensei you can get along to grade them
2. do enough multiple attacks so that they are doing techniques when they tired, and use fresh ukes.
3. do randori using ANY technique. (I once saw a high grade uke jump up at someone grading to get a strangle hold using his legs).
Although many of these result in mistakes and problems, it does help to develop the feeling of calmness in the person grading, and to forget the mistake just made and move on to a new technique.
For lower grades it is difficult. However my sensei always made sure that the attack followed him calling out the attack and defence in japanese so the student must know exactly what it is called and must also be able to react quickly. If you use a syllabus, cover certain bits of it but always ask for a few techniques that aren't on the syllabus. This may seem cruel but it will indicate one of three things:
1. student has picked up more in the lessons than you realised and is not just doing aikido to reach black belt.
2. student can't do that technique, but will improvise a technique based on the techniques he knows, showing that; if all else fails he can still do Aikido.
3. student does nothing (possibly resulting on being bonked on the head), and therefore fails his grading.
Our sensei used to change from a pleasant man into a grim old japanese speaking beast when gradings occured.
Hope this helps - if all else fails, judge them on the length of ukemi they can do!