857: Isolate JUnit4 utility code

Task-Url: https://github.com/eclipse-mylyn/org.eclipse.mylyn/issues/issues/857

Author: gnl42

mylyn.commons/org.eclipse.mylyn.commons.sdk.util/src/org/eclipse/mylyn/commons/sdk/util/MylynResourceMissingException.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2025 George
+ * Copyright (c) 2025 George Lindholm
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License v2.0
  * which accompanies this distribution, and is available at
@@ -12,21 +12,17 @@
 package org.eclipse.mylyn.commons.sdk.util;
 
 /**
- * Should really be ResourceMissingException but that could break existing code. <br/>
+ * FIXME: Should really be ResourceMissingException but that could break existing code. <br/>
  * Use to be abused junit.framework.AssertionFailedError
+ * 
+ * @Since 4.8.0
  */
 public class MylynResourceMissingException extends AssertionError {
 
 	private static final long serialVersionUID = 1L;
 
 	/**
-	 * Constructs a new AssertionFailedError without a detail message.
-	 */
-	public MylynResourceMissingException() {
-	}
-
-	/**
-	 * Constructs a new AssertionFailedError with the specified detail message. A null message is replaced by an empty String.
+	 * Constructs a new MylynResourceMissingException with the specified detail message. A null message is replaced by an empty String.
 	 *
 	 * @param message
 	 *            the detail message. The detail message is saved for later retrieval by the {@code Throwable.getMessage()} method.

mylyn.tasks/org.eclipse.mylyn.tests.util/src/org/eclipse/mylyn/tests/util/TestUtil.java

@@ -20,8 +20,8 @@
 import java.io.IOException;
 import java.util.Properties;
 
-import org.eclipse.mylyn.commons.sdk.util.MylynResourceMissingException;
 import org.eclipse.mylyn.commons.sdk.util.CommonTestUtil;
+import org.eclipse.mylyn.commons.sdk.util.MylynResourceMissingException;
 
 
 /**
@@ -80,7 +80,7 @@ public static Credentials readCredentials(PrivilegeLevel level, String realm) {
 				try {
 					file = getFile(TestUtil.class, "credentials.properties");
 					if (!file.exists()) {
-						throw new MylynResourceMissingException();
+						throw new MylynResourceMissingException("Can't find 'credentials.properties'");
 					}
 				} catch (MylynResourceMissingException e) {
 					file = new File(new File(System.getProperty("user.home"), ".mylyn"), "credentials.properties");