[9.18] Fix check interaction in the "serve-stale" test

Commit 2eee1e17 modified the "serve-stale" system test by adding the ns3/named9.conf.in configuration file and making the ns3 named instance load that file near the end of the test. However, ns3/named9.conf.in changes the stale-answer-client-timeout setting to a very low value, which affects all subsequent checks in tests.sh (rather than just the check that needs the low value to be set) and may cause false positives. Fix by reloading configuration from ns3/named8.conf.in as soon as the check using a very low stale-answer-client-timeout value is finished.

Closes #4334 (closed)