TESTS: Fix Random Fail in MockTcpTransportTests (#33061) (#33307)

* `foobar.txGet()` appears to return before `serviceB.stop()` returns, causing `ServiceB.close()` to run concurrently with the `stop` call and running into a race codition
* Closes #32863

Author: original-brownbear

test/framework/src/main/java/org/elasticsearch/transport/AbstractSimpleTransportTestCase.java

@@ -764,6 +764,7 @@ public void onAfter() {
 
     public void testNotifyOnShutdown() throws Exception {
         final CountDownLatch latch2 = new CountDownLatch(1);
+        final CountDownLatch latch3 = new CountDownLatch(1);
         try {
             serviceA.registerRequestHandler("internal:foobar", StringMessageRequest::new, ThreadPool.Names.GENERIC,
                 (request, channel) -> {
@@ -773,6 +774,8 @@ public void testNotifyOnShutdown() throws Exception {
                         serviceB.stop();
                     } catch (Exception e) {
                         fail(e.getMessage());
+                    } finally {
+                        latch3.countDown();
                     }
                 });
             TransportFuture<TransportResponse.Empty> foobar = serviceB.submitRequest(nodeA, "internal:foobar",
@@ -784,6 +787,7 @@ public void testNotifyOnShutdown() throws Exception {
             } catch (TransportException ex) {
 
             }
+            latch3.await();
         } finally {
             serviceB.close(); // make sure we are fully closed here otherwise we might run into assertions down the road
             serviceA.disconnectFromNode(nodeB);