core: add unit test ResponseTest

Prior to this fix
```
java.lang.IllegalArgumentException: Class com.cloud.agent.api.CheckS2SVpnConnectionsAnswer declares multiple JSON fields named 'details'; conflict is caused by fields com.cloud.agent.api.CheckS2SVpnConnectionsAnswer#details and com.cloud.agent.api.Answer#details
	at com.cloud.agent.transport.ResponseTest.testCheckS2SVpnConnectionsAnswer(ResponseTest.java:42)
```

Author: weizhouapache

core/src/main/java/com/cloud/agent/api/CheckS2SVpnConnectionsAnswer.java

@@ -25,7 +25,6 @@
 public class CheckS2SVpnConnectionsAnswer extends Answer {
     Map<String, Boolean> ipToConnected;
     Map<String, String> ipToDetail;
-    String details;
 
     protected CheckS2SVpnConnectionsAnswer() {
         ipToConnected = new HashMap<String, Boolean>();

core/src/test/java/com/cloud/agent/transport/ResponseTest.java

@@ -0,0 +1,46 @@
+//
+// 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.
+//
+
+package com.cloud.agent.transport;
+
+import junit.framework.TestCase;
+
+import org.apache.logging.log4j.Logger;
+import org.apache.logging.log4j.LogManager;
+import org.junit.Assert;
+import com.cloud.agent.api.Answer;
+import com.cloud.agent.api.CheckS2SVpnConnectionsAnswer;
+
+import com.cloud.agent.transport.Request.Version;
+
+public class ResponseTest extends TestCase {
+    protected Logger logger = LogManager.getLogger(getClass());
+
+    public void testCheckS2SVpnConnectionsAnswer() {
+        logger.info("Testing CheckS2SVpnConnectionsAnswer");
+        String content = "[{\"com.cloud.agent.api.CheckS2SVpnConnectionsAnswer\":{\"ipToConnected\":{\"10.0.53.13\":true}," +
+                "\"ipToDetail\":{\"10.0.53.13\":\"IPsec SA found;Site-to-site VPN have connected\"}," +
+                "\"details\":\"10.0.53.13:0:IPsec SA found;Site-to-site VPN have connected\\u0026\\n\"," +
+                "\"result\":true,\"contextMap\":{},\"wait\":0,\"bypassHostMaintenance\":false}}]";
+        Response response = new Response(Version.v2, 1L, 2L, 3L, 1L, (short)1, content);
+        Answer answer = response.getAnswer();
+        Assert.assertTrue(answer instanceof CheckS2SVpnConnectionsAnswer);
+    }
+
+}