[ML] Increase assertBusy timeout in ML node failure tests (#52425)

Following the change to store cluster state in Lucene indices
(#50907) it can take longer for all the cluster state updates
associated with node failure scenarios to be processed during
internal cluster tests where several nodes all run in the same
JVM.

x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/integration/MlDistributedFailureIT.java

@@ -451,6 +451,10 @@ private void run(String jobId, CheckedRunnable<Exception> disrupt) throws Except
         // else.
         persistentTasksClusterService.setRecheckInterval(TimeValue.timeValueMillis(200));
 
+        // The timeout here was increased from 10 seconds to 20 seconds in response to the changes in
+        // https://github.com/elastic/elasticsearch/pull/50907 - now that the cluster state is stored
+        // in a Lucene index it can take a while to update when there are many updates in quick
+        // succession, like we see in internal cluster tests of node failure scenarios
         assertBusy(() -> {
             ClusterState clusterState = client().admin().cluster().prepareState().get().getState();
             PersistentTasksCustomMetaData tasks = clusterState.metaData().custom(PersistentTasksCustomMetaData.TYPE);
@@ -471,7 +475,7 @@ private void run(String jobId, CheckedRunnable<Exception> disrupt) throws Except
                     .getResponse().results().get(0);
             assertEquals(DatafeedState.STARTED, datafeedStats.getDatafeedState());
             assertNotNull(datafeedStats.getNode());
-        });
+        }, 20, TimeUnit.SECONDS);
 
         long numDocs2 = randomIntBetween(2, 64);
         long now2 = System.currentTimeMillis();