JENKINS-8702 unit tests improvements

hudson.UDPBroadcastThread#testMulticast hangs when the network isn't enabled on my Linux box: joinGroup throws IOException and the ready lock isn't released.
This ready lock is only used in the tests.

core/src/main/java/hudson/UDPBroadcastThread.java

@@ -61,10 +61,13 @@ public class UDPBroadcastThread extends Thread {
     @Override
     public void run() {
         try {
+          try{
             mcs.joinGroup(MULTICAST);
+          } finally{
             ready.signal();
+          }
 
-            while(true) {
+          while(true) {
                 byte[] buf = new byte[2048];
                 DatagramPacket p = new DatagramPacket(buf,buf.length);
                 mcs.receive(p);

test/src/test/java/hudson/UDPBroadcastThreadTest.java

@@ -17,6 +17,8 @@ import java.io.IOException;
  * @author Kohsuke Kawaguchi
  */
 public class UDPBroadcastThreadTest extends HudsonTestCase {
+    // FIXME these tests will fail if don't have a network link
+
     /**
      * Old unicast based clients should still be able to receive some reply,
      * as we haven't changed the port.