Fix race in http2 test (netty#12547)

Motivation:

As we access the handler directly we need to ensure we wait until its added to the pipeline to eliminate races.

Modification:

Add latch to ensure we wait until the handlers are added

Result:

No more race in test

codec-http2/src/test/java/io/netty/handler/codec/http2/InboundHttp2ToHttpAdapterTest.java

@@ -96,6 +96,7 @@ public class InboundHttp2ToHttpAdapterTest {
     private CountDownLatch serverLatch2;
     private CountDownLatch clientLatch2;
     private CountDownLatch settingsLatch;
+    private CountDownLatch clientHandlersAddedLatch;
     private int maxContentLength;
     private HttpResponseDelegator serverDelegator;
     private HttpResponseDelegator clientDelegator;
@@ -663,6 +664,7 @@ private void boostrapEnv(int clientLatchCount, int clientLatchCount2, int server
         serverLatch2 = new CountDownLatch(serverLatchCount2);
         clientLatch2 = new CountDownLatch(clientLatchCount2);
         settingsLatch = new CountDownLatch(settingsLatchCount);
+        clientHandlersAddedLatch = new CountDownLatch(1);
 
         sb = new ServerBootstrap();
         cb = new Bootstrap();
@@ -735,6 +737,12 @@ public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exc
                         }
                     }
                 });
+                p.addLast(new ChannelInboundHandlerAdapter() {
+                    @Override
+                    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
+                        clientHandlersAddedLatch.countDown();
+                    }
+                });
             }
         });
 
@@ -745,6 +753,7 @@ public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exc
         clientChannel = ccf.channel();
         assertTrue(prefaceWrittenLatch.await(5, SECONDS));
         assertTrue(serverChannelLatch.await(5, SECONDS));
+        assertTrue(clientHandlersAddedLatch.await(5, SECONDS));
     }
 
     private void cleanupCapturedRequests() {