Don't servfail on staleonly lookups

When a staleonly lookup doesn't find a satisfying answer, it should
not try to respond to the client.

This is not true when the initial lookup is staleonly (that is when
'stale-answer-client-timeout' is set to 0), because no resolver fetch
has been created at this point. In this case continue with the lookup
normally.

(cherry picked from commit f8b7b597e9)

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

@@ -1069,6 +1069,16 @@ status=$((status+ret))
 
 sleep 2
 
+# Check that if we don't have stale data for a domain name, we will
+# not answer anything until the resolver query timeout.
+n=$((n+1))
+echo_i "check notincache.example times out (max-stale-ttl default) ($n)"
+ret=0
+$DIG -p ${PORT} +tries=1 +timeout=3  @10.53.0.3 notfound.example TXT > dig.out.test$n 2>&1
+grep "connection timed out" dig.out.test$n > /dev/null || ret=1
+if [ $ret != 0 ]; then echo_i "failed"; fi
+status=$((status+ret))
+
 echo_i "sending queries for tests $((n+1))-$((n+4))..."
 $DIG -p ${PORT} @10.53.0.3 data.example TXT > dig.out.test$((n+1)) &
 $DIG -p ${PORT} @10.53.0.3 othertype.example CAA > dig.out.test$((n+2)) &