Set working directory for all cpp unit tests to the /tests directory.

All unit test executables go into /tests, but their working directory was CMAKE_CURRENT_BINARY_DIR (default for add_test). This meant that when doing "include(CommonCTest)" from a subfolder (tests/cpp/CMakeLists.txt) they had as working directory tests/cpp/ and couldn't access test resources installed in /tests. It is of course possible to work around this by installing the ressources into tests/cpp, but I think it is more intuitive that the working directory corresponds to the executable location.