[TEST] Wait for replicas before stopping nodes in ML distributed test (elastic#43622)

If we stop a node before replicas exist then
the test can fail because we lose a whole
index if we stop the node with the primary on.

Author: droberts195

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

@@ -146,7 +146,9 @@ public void testCloseUnassignedJobAndDatafeed() throws Exception {
         waitForDatafeed(jobId, numDocs1);
 
         // stop the only ML node
+        ensureGreen(); // replicas must be assigned, otherwise we could lose a whole index
         internalCluster().stopRandomNonMasterNode();
+        ensureStableCluster(1);
 
         // Job state is opened but the job is not assigned to a node (because we just killed the only ML node)
         GetJobsStatsAction.Request jobStatsRequest = new GetJobsStatsAction.Request(jobId);
@@ -198,7 +200,7 @@ public void testJobRelocationIsMemoryAware() throws Exception {
 
         // Wait for the cluster to be green - this means the indices have been replicated.
 
-        ensureGreen(".ml-config", ".ml-anomalies-shared", ".ml-notifications");
+        ensureGreen();
 
         // Open a big job.  This should go on a different node to the 4 small ones.