Reproducer for CVE-2023-2911

Reproducer for CVE-2023-2911

The conditions that trigger the crash:
- a stale record is in cache
- stale-answer-client-timeout is 0
- multiple clients query for the stale record, enough of them to exceed
  the recursive-clients quota
- the response from the authoritative is sufficiently delayed so that
  recursive-clients quota is exceeded first

The reproducer attempts to simulate this situation. However, it hasn't
proven to be 100 % reproducible, especially in CI. When reproducing
locally, the priming query also seems to sometimes interfere and prevent
the crash. When the reproducer is ran twice, it appears to be more
reliable in reproducing the issue.