Add unit test for device message serialization

There was a test for the deserialization, but not for the serialization.

server/src/test/java/com/genymobile/scrcpy/DeviceMessageWriterTest.java

@@ -0,0 +1,35 @@
+package com.genymobile.scrcpy;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.io.ByteArrayOutputStream;
+import java.io.DataOutputStream;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+
+public class DeviceMessageWriterTest {
+
+    @Test
+    public void testSerializeClipboard() throws IOException {
+        DeviceMessageWriter writer = new DeviceMessageWriter();
+
+        String text = "aéûoç";
+        byte[] data = text.getBytes(StandardCharsets.UTF_8);
+        ByteArrayOutputStream bos = new ByteArrayOutputStream();
+        DataOutputStream dos = new DataOutputStream(bos);
+        dos.writeByte(DeviceMessage.TYPE_CLIPBOARD);
+        dos.writeShort(data.length);
+        dos.write(data);
+
+        byte[] expected = bos.toByteArray();
+
+        DeviceMessage msg = DeviceMessage.createClipboard(text);
+        bos = new ByteArrayOutputStream();
+        writer.writeTo(msg, bos);
+
+        byte[] actual = bos.toByteArray();
+
+        Assert.assertArrayEquals(expected, actual);
+    }
+}