Fix flaky test from #6562/#6563 (#6628)

Fix flaky test from #6562
Disable ipv6 test for #6624

jetty-server/src/test/java/org/eclipse/jetty/server/HttpOutputTest.java

@@ -27,9 +27,7 @@
 import java.nio.channels.ReadableByteChannel;
 import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
-import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
-import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 import javax.servlet.AsyncContext;
 import javax.servlet.ServletException;
@@ -830,7 +828,7 @@ public void handle(String target, Request baseRequest, HttpServletRequest reques
     @Test
     public void testEmptyBuffer() throws Exception
     {
-        AtomicBoolean committed = new AtomicBoolean();
+        FuturePromise<Boolean> committed = new FuturePromise<>();
         AbstractHandler handler = new AbstractHandler()
         {
             @Override
@@ -839,21 +837,21 @@ public void handle(String target, Request baseRequest, HttpServletRequest reques
                 baseRequest.setHandled(true);
                 response.setStatus(200);
                 ((HttpOutput)response.getOutputStream()).write(ByteBuffer.wrap(new byte[0]));
-                committed.set(response.isCommitted());
+                committed.succeeded(response.isCommitted());
             }
         };
 
         _swap.setHandler(handler);
         handler.start();
         String response = _connector.getResponse("GET / HTTP/1.0\nHost: localhost:80\n\n");
         assertThat(response, containsString("HTTP/1.1 200 OK"));
-        assertThat(committed.get(), is(false));
+        assertThat(committed.get(10, TimeUnit.SECONDS), is(false));
     }
 
     @Test
     public void testEmptyBufferWithZeroContentLength() throws Exception
     {
-        CountDownLatch latch = new CountDownLatch(1);
+        FuturePromise<Boolean> committed = new FuturePromise<>();
         AbstractHandler handler = new AbstractHandler()
         {
             @Override
@@ -863,8 +861,7 @@ public void handle(String target, Request baseRequest, HttpServletRequest reques
                 response.setStatus(200);
                 response.setContentLength(0);
                 ((HttpOutput)response.getOutputStream()).write(ByteBuffer.wrap(new byte[0]));
-                assertThat(response.isCommitted(), is(true));
-                latch.countDown();
+                committed.succeeded(response.isCommitted());
             }
         };
 
@@ -873,7 +870,7 @@ public void handle(String target, Request baseRequest, HttpServletRequest reques
         String response = _connector.getResponse("GET / HTTP/1.0\nHost: localhost:80\n\n");
         assertThat(response, containsString("HTTP/1.1 200 OK"));
         assertThat(response, containsString("Content-Length: 0"));
-        assertThat(latch.await(3, TimeUnit.SECONDS), is(true));
+        assertThat(committed.get(10, TimeUnit.SECONDS), is(true));
     }
 
     @Test