I'd have to disagree with the majority of the responses. I think testing is an important part of ones growth in the art. Especially if someone is terrified of testing, they should be encouraged to undertake the challenge.
The dojo is a safe place where people should face their fears not avoid them. Granted there are those who fear testing and are infact paralized by the thought, but that's the point. Training should be more than just twisting wrists it should be about personal and spiritual growth. To avoid testing due to fear is working in exactly the opposite direction the person should be going.
I'd suggest encouraging them to test when they feel confident, or when you know they can pass the test. Having them work with a good and senior uke on test techniques for preparation could help assuage some fears, but there will come a time where they must face their fear, and on the mat is the perfect place to do that.
Giving them the confidence to face their fears on the mat can be rolled into learning to face that fear that we have in the outside world. I think that's what O'sensei wanted aikido to be, a vehicle for self imporvement and growth, not only on the mat but also in the real world.