CI once found a test worker still running 15 s after SIGTERM, and the
failure said nothing more. The worker children now dump their threads'
stacks on SIGUSR1, and tearDown sends it to a child that is still alive
before killing it, so the next such failure names the place.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>