Use SPDX license header on new test file

New files in this project use SPDX headers attributing copyright to the
contributor, rather than the legacy Mirth Corporation header carried by
pre-fork files.

Signed-off-by: Giovanni Giannola <giogiannola@globalesm.com>

Author: ggiannola

donkey/src/test/java/com/mirth/connect/donkey/test/MessageTest.java

@@ -1,11 +1,5 @@
-/*
- * Copyright (c) Mirth Corporation. All rights reserved.
- * 
- * http://www.mirthcorp.com
- * 
- * The software in this package is published under the terms of the MPL license a copy of which has
- * been included with this distribution in the LICENSE.txt file.
- */
+// SPDX-License-Identifier: MPL-2.0
+// SPDX-FileCopyrightText: 2026 Giovanni Giannola <giogiannola@globalesm.com>
 
 package com.mirth.connect.donkey.test;