Make building tests an option in CMake

[FabienPean]

Tests were built by default with CMake, it is now an option set to ON by default to preserve previous behavior.

[fghoussen]

Can you put the previous code into a cmake function and call that function if the option is ON

[FabienPean]

Is that the change you meant (should the function be defined there) ? It looks a bit weird, did you mean refactor the previous code to follow the "Examples" part of the CMakeLists instead ?

[fghoussen]

Would be good to have equivalent of function(examples list_name) and function(pexamples list_name) but not sure if this can be easy done for all tests.

[FabienPean]

    enable_testing()
    set(CMAKE_CTEST_COMMAND ctest -V)   

had to be moved in global scope of the script because the CMake function creates a new scope. To keep it fully apart, it should be using the macro/endmacro couple instead of the function/endfunction.

Now with regard to the CI, for the failing job macos_latest_cmake_python, it looks like a problem resulting from the transition to macOS 12.

[fghoussen]

CI seems broken for other reasons: we'll have to fix it in a dedicated PR before merging this.