Add unit test for LinkedHashMap serialization

This commit adds a unit test for LinkedHashMap serialization that tests
that the method of serialization writes the entries in the LinkedHashMap
in iteration order and that the reconstructed LinkedHashMap preserves
that order. This test is randomized and tests iteration order is
preserved whether the LinkedHashMap is ordered by insertion order or
access order.

Closes #14743

core/src/test/java/org/elasticsearch/common/io/stream/StreamTests.java

@@ -25,8 +25,7 @@ import org.elasticsearch.test.ESTestCase;
 
 import java.io.IOException;
 import java.nio.ByteBuffer;
-import java.util.Arrays;
-import java.util.List;
+import java.util.*;
 
 public class StreamTests extends ESTestCase {
     public void testRandomVLongSerialization() throws IOException {
@@ -59,4 +58,34 @@ public class StreamTests extends ESTestCase {
             assertEquals(Arrays.toString(value.v2()), (long)value.v1(), bytes.streamInput().readZLong());
         }
     }
+
+    public void testLinkedHashMap() throws IOException {
+        int size = randomIntBetween(1, 1024);
+        boolean accessOrder = randomBoolean();
+        List<Tuple<String, Integer>> list = new ArrayList<>(size);
+        LinkedHashMap<String, Integer> write = new LinkedHashMap<>(size, 0.75f, accessOrder);
+        for (int i = 0; i < size; i++) {
+            int value = randomInt();
+            list.add(new Tuple<>(Integer.toString(i), value));
+            write.put(Integer.toString(i), value);
+        }
+        if (accessOrder) {
+            // randomize access order
+            Collections.shuffle(list, random());
+            for (Tuple<String, Integer> entry : list) {
+                // touch the entries to set the access order
+                write.get(entry.v1());
+            }
+        }
+        BytesStreamOutput out = new BytesStreamOutput();
+        out.writeGenericValue(write);
+        LinkedHashMap<String, Integer> read = (LinkedHashMap<String, Integer>)out.bytes().streamInput().readGenericValue();
+        assertEquals(size, read.size());
+        int index = 0;
+        for (Map.Entry<String, Integer> entry : read.entrySet()) {
+            assertEquals(list.get(index).v1(), entry.getKey());
+            assertEquals(list.get(index).v2(), entry.getValue());
+            index++;
+        }
+    }
 }