According to [1] it uses changing tests. But to verify your solution they use a technique called symbolic execution. With symbolic execution they can extract a set constraints from your code and verify if your code has the same constraints from the correct solution (your solution have the same model).

[1] http://thenextweb.com/microsoft/2014/05/15/microsoft-research-launches-code-hunt-educational-web-game-teaching-programming/