Mute UnicastZenPingTests#testSimplePings with @AwaitsFix.

This failure is being tracked in #28685.

server/src/test/java/org/elasticsearch/discovery/zen/UnicastZenPingTests.java

@@ -137,6 +137,7 @@ public class UnicastZenPingTests extends ESTestCase {
 
     private static final UnicastHostsProvider EMPTY_HOSTS_PROVIDER = Collections::emptyList;
 
+    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/28685")
     public void testSimplePings() throws IOException, InterruptedException, ExecutionException {
         // use ephemeral ports
         final Settings settings = Settings.builder().put("cluster.name", "test").put(TcpTransport.PORT.getKey(), 0).build();