Mark the autosign system test as flaky
The autosign test uses sleep in many cases to wait for something to happen. This inevitably leads to an instability that manifests in our CI. Allow an automatic rerun of the test to improve its stability.
examples of recent timing failures:
- https://gitlab.isc.org/isc-projects/bind9/-/jobs/4123353
- https://gitlab.isc.org/isc-projects/bind9/-/jobs/4123339
Related !8239 (merged)
Closes #4247 (closed)
Closes #1565 (closed)