[tests] restructure tests directory
Move io and lib related tests into io/ and lib/.
Our test dictionaries not must have a 'source' key with an array created with files(). This ensures we use always correct paths regardless of the actual directory we are currently in.
Until https://github.com/mesonbuild/meson/issues/8608 is not solved we have to separately specify the tests name in a 'name' key.