[SPARK-40096][CORE][TESTS][FOLLOW-UP] Explicitly check the element and length

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

This PR is a followup of #37533 that works around the test failure by explicitly checking the element and length in the test.

### Why are the changes needed?

For an unknown reason the test added in is flaky even though both `ArrayBuffer` and `List` are `Sep` and the test should pass up to my best knowledge. See https://github.com/apache/spark/actions/runs/3109851954/jobs/5040465291

```
[info] - SPARK-40096: Send finalize events even if shuffle merger blocks indefinitely with registerMergeResults is false *** FAILED *** (90 milliseconds)
[info]   ArrayBuffer("hostB") did not equal List("hostB") (DAGSchedulerSuite.scala:4498)
[info]   org.scalatest.exceptions.TestFailedException:
[info]   at org.scalatest.Assertions.newAssertionFailedException(Assertions.scala:472)
[info]   at org.scalatest.Assertions.newAssertionFailedException$(Assertions.scala:471)
[info]   at org.scalatest.Assertions$.newAssertionFailedException(Assertions.scala:1231)
[info]   at org.scalatest.Assertions$AssertionsHelper.macroAssert(Assertions.scala:1295)
[info]   at org.apache.spark.scheduler.DAGSchedulerSuite.$anonfun$new$286(DAGSchedulerSuite.scala:4498)
[info]   at org.scalatest.OutcomeOf.outcomeOf(OutcomeOf.scala:85)
[info]   at org.scalatest.OutcomeOf.outcomeOf$(OutcomeOf.scala:83)
[info]   at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104)
[info]   at org.scalatest.Transformer.apply(Transformer.scala:22)
[info]   at org.scalatest.Transformer.apply(Transformer.scala:20)
[info]   at org.scalatest.funsuite.AnyFunSuiteLike$$anon$1.apply(AnyFunSuiteLike.scala:226)
[info]   at org.apache.spark.SparkFunSuite.withFixture(SparkFunSuite.scala:207)
[info]   at org.scalatest.funsuite.AnyFunSuiteLike.invokeWithFixture$1(AnyFunSuiteLike.scala:224)
[info]   at org.scalatest.funsuite.AnyFunSuiteLike.$anonfun$runTest$1(AnyFunSuiteLike.scala:236)
[info]   at org.scalatest.SuperEngine.runTestImpl(Engine.scala:306)
[info]   at org.scalatest.funsuite.AnyFunSuiteLike.runTest(AnyFunSuiteLike.scala:236)
[info]   at org.scalatest.funsuite.AnyFunSuiteLike.runTest$(AnyFunSuiteLike.scala:218)
[info]   at org.apache.spark.SparkFunSuite.org$scalatest$BeforeAndAfterEach$$super$runTest(SparkFunSuite.scala:66)
[info]   at org.scalatest.BeforeAndAfterEach.runTest(BeforeAndAfterEach.scala:234)
[info]   at org.scalatest.BeforeAndAfterEach.runTest$(BeforeAndAfterEach.scala:227)
[info]   at org.apache.spark.SparkFunSuite.runTest(SparkFunSuite.scala:66)
[info]   at org.scalatest.funsuite.AnyFunSuiteLike.$anonfun$runTests$1(AnyFunSuiteLike.scala:269)
[info]   at org.scalatest.SuperEngine.$anonfun$runTestsInBranch$1(Engine.scala:413)
[info]   at scala.collection.immutable.List.foreach(List.scala:431)
```

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

No, test-only.

### How was this patch tested?

CI in this PR should verify that.

Closes #37989 from HyukjinKwon/SPARK-40096-followup.

Authored-by: Hyukjin Kwon <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: HyukjinKwon

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

@@ -4495,7 +4495,8 @@ class DAGSchedulerSuite extends SparkFunSuite with TempLocalSparkContext with Ti
       sendRequestsLatch.await()
       verify(blockStoreClient, times(2))
         .finalizeShuffleMerge(any(), any(), any(), any(), any())
-      assert(sentHosts === Seq("hostB"))
+      assert(sentHosts.nonEmpty)
+      assert(sentHosts.head === "hostB" && sentHosts.length == 1)
       completeLatch.await()
       assert(hostAInterrupted)
     }