Fix handling of stop stopping the stopping task.

[ioquatix]

Fixes https://github.com/socketry/async/issues/158.

It was possible to raise Stop on a different fiber, and if that fiber was a parent of the task that was invoking stop_children, it was possible to kill that task mid-iteration, which would prevent stopping the remaining children.

Types of Changes

• Bug fix.

Contribution

• [x] I added tests for my changes.
• [x] I tested my changes locally.
• [x] I agree to the Developer's Certificate of Origin 1.1.