[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.
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