[SPARK-31784][CORE][TEST] Fix test BarrierTaskContextSuite."share messages with allGather() call"

### What changes were proposed in this pull request?

Change from `messages.toList.iterator` to `Iterator.single(messages.toList)`.

### Why are the changes needed?

In this test, the expected result of `rdd2.collect().head` should actually be `List("0", "1", "2", "3")` but is `"0"` now.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Updated test.

Thanks WeichenXu123 reported this problem.

Closes #28596 from Ngone51/fix_allgather_test.

Authored-by: yi.wu <[email protected]>
Signed-off-by: Xingbo Jiang <[email protected]>

Author: Ngone51

core/src/test/scala/org/apache/spark/scheduler/BarrierTaskContextSuite.scala

@@ -69,12 +69,12 @@ class BarrierTaskContextSuite extends SparkFunSuite with LocalSparkContext with
       // Pass partitionId message in
       val message: String = context.partitionId().toString
       val messages: Array[String] = context.allGather(message)
-      messages.toList.iterator
+      Iterator.single(messages.toList)
     }
-    // Take a sorted list of all the partitionId messages
-    val messages = rdd2.collect().head
-    // All the task partitionIds are shared
-    for((x, i) <- messages.view.zipWithIndex) assert(x.toString == i.toString)
+    val messages = rdd2.collect()
+    // All the task partitionIds are shared across all tasks
+    assert(messages.length === 4)
+    assert(messages.forall(_ == List("0", "1", "2", "3")))
   }
 
   test("throw exception if we attempt to synchronize with different blocking calls") {