Simplify DisableOnDebug and ParentRunner

 - DisableOnDebug: merge two if branches with the same implementation
   (`return true` in the both branches).
 - Use `Collections.singletonList` instead of `Arrays.asList` since it
   contains only one item.

src/main/java/org/junit/rules/DisableOnDebug.java

@@ -103,9 +103,7 @@ public Statement apply(Statement base, Description description) {
      */
     private static boolean isDebugging(List<String> arguments) {
         for (final String argument : arguments) {
-            if ("-Xdebug".equals(argument)) {
-                return true;
-            } else if (argument.startsWith("-agentlib:jdwp")) {
+            if ("-Xdebug".equals(argument) || argument.startsWith("-agentlib:jdwp")) {
                 return true;
             }
         }

src/main/java/org/junit/runners/ParentRunner.java

@@ -65,7 +65,7 @@
  */
 public abstract class ParentRunner<T> extends Runner implements Filterable,
         Orderable {
-    private static final List<TestClassValidator> VALIDATORS = Arrays.<TestClassValidator>asList(
+    private static final List<TestClassValidator> VALIDATORS = Collections.<TestClassValidator>singletonList(
             new AnnotationsValidator());
 
     private final Lock childrenLock = new ReentrantLock();