I think it's caused by your xcode version. Apple remove the library code-coverage needed after xcode 4.3. You need to add one file to your project manually. You can see check this blog for more details: http://www.gerardcondon.com/blog/2012/02/21/code-coverage-updates-for-xcode-4-dot-3/
You can try this. I tried but failed since my project include C++ code. I think that's because xcode 4.6 has not supported this feature...