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.

Futures can have a registered Callback of type
std::function<void(const uint32_t&)> which gets called in a new
Fiber with the result of the IO Request.
Note the first completion will cause the execution of a callback and
partial completion support must be implemented manually in the callback.

Callbacks are stored in a heap allocated std::function on registration
and are freed by the new Fiber after the callback returned;

The Future with a registered Callback is not referenced in any way in
the IO subsystem and therefore can be dropped after being submitted.
This also means that a Future with a registered callback will not be
signaled by the IO subsystem on completion.
If signaling is desired one must implement it manually in the registered
callback.