Add islands of trust to mkeys test

This adds an island of trust that is reachable from the root
where the trust anchors are added to island.conf.

This add an island of trust that is not reachable from the root
where the trust anchors are added to private.conf.

bin/tests/system/mkeys/ns5/foo.db

@@ -0,0 +1,23 @@
+; 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 20
+foo.                   IN SOA  marka.isc.org. ns.foo. (
+                               2000042100      ; serial
+                               600             ; refresh
+                               600             ; retry
+                               1200            ; expire
+                               2               ; minimum
+                               )
+foo.		       NS      ns.foo.
+ns.foo.		       A       10.53.0.5
+sub.foo.	       NS      ns.sub.foo.
+ns.sub.foo.	       A       10.53.0.4