I hate coding tests for a job interview. I've done three types:
1) university style of "write function that does x" (fizz buzz)
2) this code has a bug, fix it
3) write a program that does this university style contrived thing (exact change)

One and two usually are timed to 10 to 15 minutes per question. Three is untimed but unstated "time counts".

Off all three, the last one is the best in my opinion. However it still seem like we could come up with something better.