moved root CMakeLists.txt tests config to tests/CMakeLists.txt

Hi there,
Just started with zmq (brilliant work, thanks for sharing), I thought it could be nicer a more modular CMakeLists.txt, but I haven't found any policy regarding its design in your info.
So I propose a small change to check if it is aligned with your criteria, just moving CTest cmake stuff to its own scope in the tests folder.
Tested with cmake 2.8.12 and VS12.
Looking forward to receiving your feedback.