Avoid false positive in serve-stale system test check

Avoid false positive in serve-stale system test check

The purpose of the check is to verify the server has survived the
previous barrage of queries. This is done by sending a query and
checking we get a NOERROR response back.

Previously, that query could've been affected by a servfail cache - the
server would return a SERVFAIL answer, thus failing the check, despite
being up and running. Use version.bind txt ch query to avoid the
interference of servfail cache.

(cherry picked from commit dd7bcd2855888969778910deaac47aa54786c877)