doth system test - decrease HTTP listener quota size

This commit reduces the size of HTTP listener quota from 300 (default)
to 100 so that it would make hitting any global limits in case of
running multiple tests in parallel in multiple containers unlikely.

This way the need in opening many file descriptors of different
kinds (e.g. client side connections and pipes) gets significantly
reduced while the required code paths are still verified.

bin/tests/system/doth/ns1/named.conf.in

@@ -19,6 +19,7 @@ controls {
 
 http local {
 	endpoints { "/dns-query"; "/alter"; };
+	listener-clients 100;
 };
 
 tls tls-forward-secrecy {