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.

bin/tests/system/serve-stale/tests.sh

@@ -1834,7 +1834,7 @@ while [ $num -lt 20 ]; do
     num=$((num+1))
 done;
 _dig_data() {
-    $DIG -p ${PORT} @10.53.0.3 data.example TXT >dig.out.test$n || return 1
+    $DIG -p ${PORT} @10.53.0.3 version.bind txt ch >dig.out.test$n || return 1
     grep "status: NOERROR" dig.out.test$n > /dev/null || return 1
 }
 retry_quiet 5 _dig_data || ret=1