Avoid file glob in CMakeLists

[mknaranja]

As The code line https://github.com/mknaranja/GMGPolar/blob/ddc23c2df1f424ced9a8c905f35dc43e8308bc3e/CMakeLists.txt#L6 is considered bad practice (https://stackoverflow.com/questions/32411963/why-is-cmake-file-glob-evil) we should go over to concrete list of files. However, this can be done easily without a lot of work by just using python to create folder lists:

import os
for file in os.listdir('src/test_cases'):
    print('X' + file)

We should then probably work with add_subdirectory that contain the corresponding CMakeLists.txt.