test.CLEANUP_POLL_INTERVAL

Sleeping rather than yielding, for the reason [design.md] gives about the async poll loops and which applies with more force here: this runs inside spawn_blocking while waiting for a *process* to exit, so a spin holds a core the daemon needs to handle the signal it was just sent. On a machine with fewer cores than the suite has concurrent fixtures, that is the difference between a clean shutdown and a grace window that expires.

[design.md]: ../docs/design.md