[ML][DataFrame] Consider data frame templates internal in REST tests (elastic#43692)

The data frame index template pattern was not in the list
considered as internal and therefore not needing cleanup
after every test.

Author: droberts195

test/framework/src/main/java/org/elasticsearch/test/rest/ESRestTestCase.java

@@ -938,6 +938,9 @@ private static boolean isXPackTemplate(String name) {
         if (name.startsWith(".watch") || name.startsWith(".triggered_watches")) {
             return true;
         }
+        if (name.startsWith(".data-frame-")) {
+            return true;
+        }
         if (name.startsWith(".ml-")) {
             return true;
         }