fix: test: Fix the rpz servfail-until-ready test when zone updates run serially

The rpz servfail-until-ready test assumed a particular policy zone always
finished loading last, which only holds when zone updates run in parallel;
on a single CPU (or with serialized offload) it could fail spuriously. It now
polls until RPZ reports ready instead.

Merge branch 'ondrej/fix-rpz-system-test-on-single-cpu' into 'main'

See merge request isc-projects/bind9!12251