gio/tests/task: Run the worker indefinitely until it's cancelled
Currently, the actual asynchronous work, represented by asynchronous_cancellation_run_task, was over before the GCancellable could be triggered. While that doesn't invalidate the purpose of the test, since it's fundamentally about cancellation, it would be nicer if the cancellation actually served some purpose instead of being a mere formality.
This was discussed in !509 (merged)