cmake list does not append to lists in the parent scope

[kidder]

The cmake command

list(APPEND A_LIST STUFF_TO_APPEND)

will only append to A_LIST in the current scope.

We need to fix this in multiple locations for adding external libraries.
The proposed fix is to define a new cmake function

append_to_list(...)

which uses the cmake function set which can modify a variable in the PARENT_SCOPE

[nilsdeppe]

We might be able to just use set(SPECTRE_TESTS "${SPECTRE_TESTS};${UTILITIES_TESTS}" PARENT_SCOPE), for example.

[kidder]

This is actually not an issue for external libraries as the Setup*.cmake files are included into the top-level CMakeLists.txt and therefore the lists are in the global scope.