"rpz" system test: fix intermittent false positive (expired RPZ zone)

The rpz system test intermittently fails the following check:

I:rpz:checking policies from expired zone are no longer in effect (20)
I:rpz:failed

This is a false positive caused by unfavorable timing. It happens particularly often for the v9_11_sub branch, where it tends to cause failures for multiple jobs per pipeline. In one of the pipelines, 5 jobs failed the rpz system test in the above way:

• https://gitlab.isc.org/isc-private/bind9/-/jobs/1268026
• https://gitlab.isc.org/isc-private/bind9/-/jobs/1268029
• https://gitlab.isc.org/isc-private/bind9/-/jobs/1268032
• https://gitlab.isc.org/isc-private/bind9/-/jobs/1268044
• https://gitlab.isc.org/isc-private/bind9/-/jobs/1268047

The rationale behind the fix can be found in the commit message.