Fix race in ClusterStatsIT

ClusterStatsIT#testClusterStatus() contained a race where the
test cluster might still be initializing while test already checks
for a green health status.

With this commit the test waits until the cluster status changed and
checks health afterwards.

Checked with @bleskes.

core/src/test/java/org/elasticsearch/action/admin/cluster/stats/ClusterStatsIT.java

@@ -171,7 +171,7 @@ public class ClusterStatsIT extends ESIntegTestCase {
         assertThat(response.getNodesStats().getOs().getAllocatedProcessors(), equalTo(7));
     }
 
-    public void testClusterStatus() throws Exception {
+    public void testClusterStatusWhenStateNotRecovered() throws Exception {
         // stop all other nodes
         internalCluster().ensureAtMostNumDataNodes(0);
 
@@ -180,6 +180,8 @@ public class ClusterStatsIT extends ESIntegTestCase {
         assertThat(response.getStatus(), equalTo(ClusterHealthStatus.RED));
 
         internalCluster().ensureAtLeastNumDataNodes(3);
+        // wait for the cluster status to settle
+        ensureGreen();
         response = client().admin().cluster().prepareClusterStats().get();
         assertThat(response.getStatus(), equalTo(ClusterHealthStatus.GREEN));
     }