Make double-closing TestHttpChannel an assertion failure (#96564)

server/src/test/java/org/elasticsearch/rest/action/RestCancellableNodeClientTests.java

@@ -225,7 +225,8 @@ public class RestCancellableNodeClientTests extends ESTestCase {
         @Override
         public void close() {
             if (open.compareAndSet(true, false) == false) {
-                throw new IllegalStateException("channel already closed!");
+                assert false : "HttpChannel is already closed";
+                return;     // nothing to do
             }
             ActionListener<Void> listener = closeListener.get();
             if (listener != null) {