Ncc/reorganize tests

[mabruzzo]

I reorganized the CMake logic in the tests subdirectory. I've thought about doing this for a long time

This is extremely simple:

1. the first commit moves everything after the first two lines of tests/CMakeLists.txt into tests/code_examples/CMakeLists.txt and adds add_subdirectory(code_examples) to tests/CMakeLists.txt
2. the second commit creates a tests/system subdirectory, to hold system-tests (aka end-2-end tests), and relocates the test_ghost_zone.cpp file (and accompanying CMake target declaration) into that new subdirectory (out of tests/unit)