test: add failing tests for the async generation guard and kill path

The tests cover a fresh result being processed, a stale one discarded without touching the replacement child's handle or the failure counter, the watchdog and chime--stop both superseding the generation, and chime--kill-async-process reaping a real child and its buffer.

Where a mock can't show the behavior, the tests spawn real processes. async.el's buffer handling and the sentinel silencing are the thing under test.

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