[MINOR][CORE][TESTS] Fix flaky DAGSchedulerSuite test for push-based shuffle

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

  Add `sc.listenerBus.waitUntilEmpty()` before the `completedStage` assertion in the push-based shuffle test in `DAGSchedulerSuite`.

  ### Why are the changes needed?

  The test was intermittently failing due to a race condition. `SparkListenerStageCompleted` events are delivered asynchronously through the
  listener bus, so the `completedStage` assertion could execute before the event was processed, causing a spurious failure. Waiting for the
  listener bus to drain before asserting eliminates the race.

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

  No.

  ### How was this patch tested?

  Existing test in `DAGSchedulerSuite`. The fix removes a race condition rather than adding new test logic.

  ### Was this patch authored or co-authored using generative AI tooling?

  Generated-by: Claude Sonnet 4.6 (Claude Code)

Closes #55221 from dbtsai/dagscheduler-flaky-fix.

Authored-by: DB Tsai <dbtsai@dbtsai.com>
Signed-off-by: DB Tsai <dbtsai@dbtsai.com>

Author: dbtsai

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

@@ -5827,6 +5827,7 @@ class DAGSchedulerSuite extends SparkFunSuite with TempLocalSparkContext with Ti
       // Map stage completes successfully,
       completeShuffleMapStageSuccessfully(0, 0, 3, Seq("hostA", "hostB"))
       taskIdCount += 2
+      sc.listenerBus.waitUntilEmpty()
       assert(completedStage === List(0))
 
       // Now submit the first reducer stage