I have a project with the following structure:
linalg
├── build
├── CMakeLists.txt
├── docs
│ └── Doxyfile
├── include
│ └── linalg
│ └── vector3.h
The crux of the problem is that enable_testing should be called from your top-level CMakeLists.txt in this case. Adding include(CTest) to your top-level CMakeLists.txt should fix this for you.
This would allow you to remove the enable_testing call in test/CMakeLists.txt, since the CTest submodule calls enable_testing internally.