I'm surprised that the whole process is based on evaluating how good the candidate can apply some TDD techniques. I'm sure that more than 99% of all software is developed without unit tests. This includes the linux kernel and the rest of the OS, the other big OS-es, databases and other commercial software. Even most of the Java code doesn't have detailed unit tests. To base your evaluation entirely on how well the candidate uses those technical seems not very objective to me. I've seen people who are very good at working with very messy code which is impossible to refactor in a reasonable time and they do a very good job without any unit tests. This skill is sometimes much more important than knowing how to run you tests and few refactoring tricks.

TDD is a methodology - not an ability. A good developer can develop the habits in very short time. If this is part of the culture people can be trained to use it.