### What changes were proposed in this pull request?
Fix the flaky test.
### Why are the changes needed?
The test is flaky: `Expected exception org.apache.spark.SparkException to be thrown, but no exception was thrown`.
Check the full error stack [here](https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/128548/testReport/org.apache.spark.scheduler/BarrierTaskContextSuite/throw_exception_if_the_number_of_barrier___calls_are_not_the_same_on_every_task/).
By analyzing the log below, I found that task 0 hadn't reached the second `context.barrier()` when another three tasks already raised the sync timeout exceptions by the first `context.barrier()`. The timeout exceptions were caught by the `try...catch...`. Then, each task started another round barrier sync from the second `context.barrier()` and completed the sync successfully.
```scala
20/09/10 20:54:48.821 dispatcher-event-loop-10 INFO BarrierCoordinator: Current barrier epoch for Stage 0 (Attempt 0) is 0.
20/09/10 20:54:48.822 dispatcher-event-loop-10 INFO BarrierCoordinator: Barrier sync epoch 0 from Stage 0 (Attempt 0) received update from Task 2, current progress: 1/4.
20/09/10 20:54:48.826 dispatcher-BlockManagerMaster INFO BlockManagerInfo: Added broadcast_0_piece0 in memory on localhost:38420 (size: 2.2 KiB, free: 546.3 MiB)
20/09/10 20:54:48.908 dispatcher-event-loop-12 INFO BarrierCoordinator: Current barrier epoch for Stage 0 (Attempt 0) is 0.
20/09/10 20:54:48.909 dispatcher-event-loop-12 INFO BarrierCoordinator: Barrier sync epoch 0 from Stage 0 (Attempt 0) received update from Task 1, current progress: 2/4.
20/09/10 20:54:48.959 dispatcher-event-loop-11 INFO BarrierCoordinator: Current barrier epoch for Stage 0 (Attempt 0) is 0.
20/09/10 20:54:48.960 dispatcher-event-loop-11 INFO BarrierCoordinator: Barrier sync epoch 0 from Stage 0 (Attempt 0) received update from Task 3, current progress: 3/4.
20/09/10 20:54:49.616 dispatcher-CoarseGrainedScheduler INFO TaskSchedulerImpl: Skip current round of resource offers for barrier stage 0 because the barrier taskSet requires 4 slots, while the total number of available slots is 0.
20/09/10 20:54:49.899 dispatcher-event-loop-15 INFO BarrierCoordinator: Current barrier epoch for Stage 0 (Attempt 0) is 0.
20/09/10 20:54:49.900 dispatcher-event-loop-15 INFO BarrierCoordinator: Barrier sync epoch 0 from Stage 0 (Attempt 0) received update from Task 1, current progress: 1/4.
20/09/10 20:54:49.965 dispatcher-event-loop-13 INFO BarrierCoordinator: Current barrier epoch for Stage 0 (Attempt 0) is 0.
20/09/10 20:54:49.966 dispatcher-event-loop-13 INFO BarrierCoordinator: Barrier sync epoch 0 from Stage 0 (Attempt 0) received update from Task 3, current progress: 2/4.
20/09/10 20:54:50.112 dispatcher-event-loop-16 INFO BarrierCoordinator: Current barrier epoch for Stage 0 (Attempt 0) is 0.
20/09/10 20:54:50.113 dispatcher-event-loop-16 INFO BarrierCoordinator: Barrier sync epoch 0 from Stage 0 (Attempt 0) received update from Task 0, current progress: 3/4.
20/09/10 20:54:50.609 dispatcher-CoarseGrainedScheduler INFO TaskSchedulerImpl: Skip current round of resource offers for barrier stage 0 because the barrier taskSet requires 4 slots, while the total number of available slots is 0.
20/09/10 20:54:50.826 dispatcher-event-loop-17 INFO BarrierCoordinator: Current barrier epoch for Stage 0 (Attempt 0) is 0.
20/09/10 20:54:50.827 dispatcher-event-loop-17 INFO BarrierCoordinator: Barrier sync epoch 0 from Stage 0 (Attempt 0) received update from Task 2, current progress: 4/4.
20/09/10 20:54:50.827 dispatcher-event-loop-17 INFO BarrierCoordinator: Barrier sync epoch 0 from Stage 0 (Attempt 0) received all updates from tasks, finished successfully.
```
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
Updated the test and tested a hundred times without failure(Previously, there could be several failures).
Closes#29732 from Ngone51/fix-flaky-throw-exception.
Authored-by: yi.wu <yi.wu@databricks.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
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