Add missing @Override annotations (#5430)

core/src/test/java/hudson/ChannelRule.java

@@ -33,11 +33,13 @@ public final class ChannelRule extends ExternalResource {
         final FastPipedOutputStream p2o = new FastPipedOutputStream(p2i);
 
         Future<Channel> f1 = executors.submit(new Callable<Channel>() {
+            @Override
             public Channel call() throws Exception {
                 return new ChannelBuilder("This side of the channel", executors).withMode(Channel.Mode.BINARY).build(p1i, p2o);
             }
         });
         Future<Channel> f2 = executors.submit(new Callable<Channel>() {
+            @Override
             public Channel call() throws Exception {
                 return new ChannelBuilder("The other side of the channel", executors).withMode(Channel.Mode.BINARY).build(p2i, p1o);
             }