[v9_16] "rpz" system test: fix intermittent false positive (expired RPZ zone)

In order for a "fast-expire/IN: response-policy zone expired" message to be logged in ns3/named.run, the "fast-expire" zone must first be transferred in by that server. However, with unfavorable timing, ns3 may be stopped before it manages to fetch the "fast-expire" zone from ns5 and after the latter has been reconfigured to no longer serve that zone. In such a case, the "rpz" system test will report a false positive for the relevant check. Prevent that from happening by ensuring ns3 manages to transfer the "fast-expire" zone before getting shut down.

(cherry picked from commit 39191052)