Fix compilation errors in ML integration tests

After elastic#29109, the `needsReassignment` method has
been moved to the PersistentTasksClusterService. This commit fixes
some compilation in tests I introduced.

Author: tlrx

server/src/main/java/org/elasticsearch/persistent/PersistentTasksClusterService.java

@@ -312,7 +312,7 @@ static boolean persistentTasksChanged(final ClusterChangedEvent event) {
     }
 
     /** Returns true if the task is not assigned or is assigned to a non-existing node */
-    static boolean needsReassignment(final Assignment assignment, final DiscoveryNodes nodes) {
+    public static boolean needsReassignment(final Assignment assignment, final DiscoveryNodes nodes) {
         return (assignment.isAssigned() == false || nodes.nodeExists(assignment.getExecutorNode()) == false);
     }