[TEST] Expect nodes getting disconnected quickly

If all nodes get disconnected before we can send the request we might
try to reconnect and that will fail with an ISE instead of the a transport
exception.

Closes #25301

core/src/test/java/org/elasticsearch/transport/RemoteClusterServiceTests.java

@@ -410,6 +410,9 @@ public class RemoteClusterServiceTests extends ESTestCase {
                             });
                         failLatch.await();
                         assertNotNull(ex.get());
+                        if (ex.get() instanceof  IllegalStateException) {
+                            assertEquals(ex.get().getMessage(), "no seed node left");
+                        } else {
                             if (ex.get() instanceof TransportException == false) {
                                 // we have an issue for this see #25301
                                 logger.error("expected TransportException but got a different one see #25301", ex.get());
@@ -422,3 +425,4 @@ public class RemoteClusterServiceTests extends ESTestCase {
             }
         }
     }
+}