PeerGroupTest: Add listeners before calling start()

PeerGroupTest.listener() currently starts the PeerGroup before adding listeners to
it. While the events being listened for are not triggered until later in
the test, it is good practice to add listeners before calling start()

core/src/test/java/org/bitcoinj/core/PeerGroupTest.java

@@ -108,9 +108,9 @@ public class PeerGroupTest extends TestWithPeerGroup {
 
     @Test
     public void listener() throws Exception {
-        peerGroup.start();
         peerGroup.addConnectionEventListener(listener);
         peerGroup.addPreMessageReceivedEventListener(preMessageReceivedListener);
+        peerGroup.start();
 
         // Create a couple of peers.
         InboundMessageQueuer p1 = connectPeer(1);