[test] Made it easier to run the tests on just one of the resource files.

test/core/src/lombok/DirectoryRunner.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2009-2015 The Project Lombok Authors.
+ * Copyright (C) 2009-2019 The Project Lombok Authors.
  * 
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
@@ -36,6 +36,8 @@ import org.junit.runner.notification.Failure;
 import org.junit.runner.notification.RunNotifier;
 
 public class DirectoryRunner extends Runner {
+	private static final String DEBUG_FOCUS_ON_FILE = null;
+	
 	public enum Compiler {
 		DELOMBOK {
 			@Override public int getVersion() {
@@ -73,10 +75,11 @@ public class DirectoryRunner extends Runner {
 		
 		public abstract boolean expectChanges(); 
 	}
-		
+	
 	private static final FileFilter JAVA_FILE_FILTER = new FileFilter() {
 		@Override public boolean accept(File file) {
-			return file.isFile() && file.getName().endsWith(".java");
+			return file.isFile() && file.getName().endsWith(".java") &&
+				(DEBUG_FOCUS_ON_FILE == null || file.getName().equals(DEBUG_FOCUS_ON_FILE));
 		}
 	};