[TEST] Fix RemoteClusterConnectionTests

With PR #31574 we now ensure that connections are established under the proper
thread context. There is a test in RemoteClusterConnectionTests, however,
that shuts down the service while connecting. With the above change, a new kind
of exception can occur that the test is unaware of.

server/src/test/java/org/elasticsearch/transport/RemoteClusterConnectionTests.java

@@ -84,6 +84,7 @@
 
 import static java.util.Collections.emptyMap;
 import static java.util.Collections.emptySet;
+import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.iterableWithSize;
@@ -860,7 +861,13 @@ public void run() {
                                                     throw new AssertionError(x);
                                                 }
                                         });
-                                        connection.updateSeedNodes(seedNodes, listener);
+                                        try {
+                                            connection.updateSeedNodes(seedNodes, listener);
+                                        } catch (Exception e) {
+                                            // it's ok if we're shutting down
+                                            assertThat(e.getMessage(), containsString("threadcontext is already closed"));
+                                            latch.countDown();
+                                        }
                                     }
                                     latch.await();
                                 } catch (Exception ex) {