The test is a simple CMake script that finds all files in the build
directory with certain extensions (.h, .cxx, etc.) and makes sure that
the filename is listed somewhere in the CMakeLists.txt file of the same
directory. If the filename is listed in the CMakeLists.txt file, then
there is a good chance it is being addressed by the build.
This should help catch when header files are not being installed. It also
should help verify that test builds are being done on all files. It will
also highlight dead source files.
