Fix testListenersNotifiedOnCorrectThreads (#68805)

This test assumed, incorrectly, that `future#done()` completes before
`future#set()` returns, but this isn't true if there are multiple
threads racing to complete the future. In other words listeners added
before calling `onResponse()` are not necessarily notified by the time
`onResponse()` returns. This commit fixes the test to account for this
subtle point.

Closes #68772

Author: DaveCTurner

server/src/test/java/org/elasticsearch/action/support/ListenableActionFutureTests.java

@@ -68,7 +68,6 @@ protected void doRun() {
         }
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/68772")
     public void testListenersNotifiedOnCorrectThreads() throws InterruptedException {
 
         final int adderThreads = between(1, 5);
@@ -90,7 +89,10 @@ public void testListenersNotifiedOnCorrectThreads() throws InterruptedException
                     awaitSafe(barrier);
 
                     final AtomicBoolean isComplete = new AtomicBoolean();
-                    if (postComplete.get()) {
+                    if (completerThreads == 1 && postComplete.get()) {
+                        // If there are multiple completer threads then onResponse might return on one thread, and hence postComplete is
+                        // set, before the other completer thread notifies all the listeners. OTOH with one completer thread we know that
+                        // postComplete indicates that the listeners were already notified.
                         future.addListener(new ActionListener<>() {
                             @Override
                             public void onResponse(Void response) {