Create a cmake project testing another cmake project

we have a cmake project which does not use automated tests. So I want to create another cmake project including the Boost-test framework to test the other project. I know, it would be perhaps better to add the test project within the other project, but we have to change the project structure for this purpose. So I want a non-invasive test project. Here is the structure of my idea:

   +---Project_A
          |
          CMakeList.txt
          src
          modules
          .
          .
          .
   +---Test_Project
          |
          CMakeList.txt

I tried to use the cmake-include statement in the Test_Project to include Project_A. But CMake tries to compile the Project_A-files within the Test_Project. So how can I tell CMake to compile the files within Project_A? Do you know maybe a better way to test Project_A without changing it's project structure?