test(gax): fix flakiness in BatcherImplTest.testElementsNotLeaking (#13572)

Increase the timeout and use Awaitility to wait for the background task
to complete. This avoids timing out on slower machines (like macOS
Intel).

Fixes #13565

Author: lqiu96

sdk-platform-java/gax-java/gax/src/test/java/com/google/api/gax/batching/BatcherImplTest.java

@@ -516,7 +516,6 @@ void testWhenDelayThresholdExceeds() throws Exception {
 
   /** Validates that the elements are not leaking to multiple batches */
   @Test
-  @Timeout(value = 500, unit = TimeUnit.MILLISECONDS)
   void testElementsNotLeaking() throws Exception {
     ExecutorService singleThreadExecutor = Executors.newSingleThreadExecutor();
     ScheduledExecutorService multiThreadExecutor = Executors.newScheduledThreadPool(20);
@@ -567,7 +566,7 @@ public ApiFuture<List<Integer>> futureCall(
       }
 
       // Closing the resources
-      future.get();
+      future.get(10, TimeUnit.SECONDS);
       assertThat(isDuplicateElement.get()).isFalse();
       singleThreadExecutor.shutdown();
       multiThreadExecutor.shutdown();