Do not reuse test cluster in CrossClusterSearchIT

The `testProxyConnectionDisconnect` test can use a node without the
remote_cluster_client that is created in `testRemoteClusterClientRole`.

Author: dnhatn

server/src/internalClusterTest/java/org/elasticsearch/search/ccs/CrossClusterSearchIT.java

@@ -59,6 +59,11 @@ protected Collection<String> remoteClusterAlias() {
         return List.of("cluster_a");
     }
 
+    @Override
+    protected boolean reuseClusters() {
+        return false;
+    }
+
     private int indexDocs(Client client, String index) {
         int numDocs = between(1, 10);
         for (int i = 0; i < numDocs; i++) {