Merge branch 'frame-base-tests' into 'master'

Ctest configuration

This replicates issue #115 , which in turn traces back to !103, regarding `ctest` and `gtest`.

I finally left the project as this (checkout branch `frame-base-tests`):

  - `wolf/src/tests` --> here all the gtest files are located

  - In `wolf/CMakeLists.txt` 

```cmake
if(BUILD_TESTS)
    enable_testing()
endif()
```

  - In `wolf/src/CMakeLists.txt`

```cmake
if(BUILD_TESTS)    
    MESSAGE("Building tests.")
    add_subdirectory(test)
endif()
```

See merge request !104