Until kernel commit cf10960426515 io_uring submitted sqe until it encountered
an invalid one. Now io_uring will catch the error on subimssion and
will cancel all sqe's contained in the link prior to the invalid one.

errno was never set on error and thus the DIE_MSG_ERRNO did not properly
report what happened.

Found during changing Future member types.

* Improve warn message in Future::cancel()
* Don't cancel a Future in the destructor if it was already cancelled or received
* Restructure CancelFutureTest into single separate test cases

With this I can not reproduce the weird CancelFutureTest behavior anymore
neither SIGABRT nor the timeout.

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.