You can add a global variable at the level of your root CMakeLists.txt:
set(ALL_SRCS CACHE INTERNAL "mydescription" FORCE)
In the first add_subdirectory(src), you can do:
set(ALL_SRCS ${ALL_SRCS} blabla.cpp CACHE INTERNAL "description")
And in the add_subdirectory(test), you continue with:
set(ALL_SRCS ${ALL_SRCS} bla_test.cpp CACHE INTERNAL "description")
You can then do, add_executable, or library or whatever, with all your sources files.
EDIT: add trick for global variables in CMake.