Implement true INTERLEAVE ordering.

Add manual gated IT to dump DEFAULT vs INTERLEAVE results.
Expand unit coverage for scope mapping and core RFC comparison rules.

Author: arturobernalg

httpclient5-testing/src/test/java/org/apache/hc/client5/testing/ManualRfc6724ResolverIT.java

@@ -0,0 +1,87 @@
+/*
+ * ====================================================================
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ * ====================================================================
+ *
+ * This software consists of voluntary contributions made by many
+ * individuals on behalf of the Apache Software Foundation.  For more
+ * information on the Apache Software Foundation, please see
+ * <http://www.apache.org/>.
+ *
+ */
+
+package org.apache.hc.client5.testing;
+
+import static org.junit.jupiter.api.Assumptions.assumeTrue;
+
+import java.net.Inet6Address;
+import java.net.InetAddress;
+
+import org.apache.hc.client5.http.DnsResolver;
+import org.apache.hc.client5.http.Rfc6724AddressSelectingDnsResolver;
+import org.apache.hc.client5.http.SystemDefaultDnsResolver;
+import org.apache.hc.client5.http.config.ProtocolFamilyPreference;
+import org.junit.jupiter.api.Test;
+
+class ManualRfc6724ResolverIT {
+
+    private static final String PROP_ENABLE = "httpclient.rfc6724.it";
+    private static final String PROP_HOST = "httpclient.rfc6724.host";
+
+    @Test
+    void resolve_and_dump_order() throws Exception {
+        assumeTrue(Boolean.getBoolean(PROP_ENABLE),
+                "Enable with -Dhttpclient.rfc6724.it=true");
+
+        final String host = System.getProperty(PROP_HOST, "localhost");
+
+        final DnsResolver base = SystemDefaultDnsResolver.INSTANCE;
+
+        System.out.println("Host: " + host);
+
+        dump(base, host, ProtocolFamilyPreference.DEFAULT);
+        System.out.println();
+        dump(base, host, ProtocolFamilyPreference.INTERLEAVE);
+    }
+
+    private static void dump(
+            final DnsResolver base,
+            final String host,
+            final ProtocolFamilyPreference pref) throws Exception {
+
+        final Rfc6724AddressSelectingDnsResolver resolver =
+                new Rfc6724AddressSelectingDnsResolver(base, pref);
+
+        final InetAddress[] out = resolver.resolve(host);
+
+        int v4 = 0;
+        int v6 = 0;
+
+        System.out.println("Preference: " + pref);
+        for (final InetAddress a : out) {
+            if (a instanceof Inet6Address) {
+                v6++;
+                System.out.println("  IPv6 " + a.getHostAddress());
+            } else {
+                v4++;
+                System.out.println("  IPv4 " + a.getHostAddress());
+            }
+        }
+        System.out.println("Counts: IPv4=" + v4 + " IPv6=" + v6);
+    }
+}