Add system test for forward only grafted zone with synth-from-dnssec

We are grafting on an unsigned zone "example.internal" where the higher
zone (".") is signed and would otherwise cause named to synthesise a
NXDOMAIN for example.internal.  We prime the cache by performing a
lookup for "internal" and then lookup "example.internal".

bin/tests/system/synthfromdnssec/ns2/example.internal.db

@@ -0,0 +1,16 @@
+; Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+;
+; SPDX-License-Identifier: MPL-2.0
+;
+; This Source Code Form is subject to the terms of the Mozilla Public
+; License, v. 2.0.  If a copy of the MPL was not distributed with this
+; file, you can obtain one at https://mozilla.org/MPL/2.0/.
+;
+; See the COPYRIGHT file distributed with this work for additional
+; information regarding copyright ownership.
+
+$TTL 3600
+@		SOA	ns2 hostmaster 1 3600 1200 604800 3600
+@		NS	ns2
+@		A	1.2.3.4
+ns2		A	10.53.0.2