[ML] testFullClusterRestart waiting for stable cluster (#46280)

* [ML] waiting for ml indices before waiting task assignment testFullClusterRestart

* waiting for a stable cluster after fullrestart

* removing unused imports

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

@@ -113,6 +113,7 @@ public void testFullClusterRestart() throws Exception {
             logger.info("Restarting all nodes");
             internalCluster().fullRestart();
             logger.info("Restarted all nodes");
+            ensureStableClusterOnAllNodes(3);
         });
     }