CCR: Test follow on top of closed index (#54956)

Added testing of following on top of a closed index.
This could for instance be the old leader index in
cases where leader and follower clusters have been
swapped.

x-pack/plugin/ccr/src/test/java/org/elasticsearch/xpack/ccr/IndexFollowingIT.java

@@ -1294,6 +1294,9 @@ public class IndexFollowingIT extends CcrIntegTestCase {
 
         followerClient().admin().indices().prepareClose("index2").get();
         pauseFollow("index2");
+        if (randomBoolean()) {
+            assertAcked(followerClient().execute(UnfollowAction.INSTANCE, new UnfollowAction.Request("index2")).actionGet());
+        }
 
         final PutFollowAction.Request followRequest2 = putFollow("index1", "index2");
         PutFollowAction.Response response2 = followerClient().execute(PutFollowAction.INSTANCE, followRequest2).get();