[FLINK-34571][test] Fix flaky test SortMergeResultPartitionReadSchedulerTest.testOnReadBufferRequestError

[reswqa]

What is the purpose of the change

We have a race condition in SortMergeResultPartitionReadSchedulerTest.testOnReadBufferRequestError, sometimes the reader finish reading before buffer pool destroyed.

Brief change log

• Use a manually trigged executor to get rid of the race condition.

Verifying this change

Please make sure both new and modified tests in this PR follows the conventions defined in our code quality guide: https://flink.apache.org/contributing/code-style-and-quality-common.html#testing

This actually fix a flaky test itself.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no

[flinkbot]

CI report:

• f73314d Azure: SUCCESS

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[WencongLiu]

LGTM. This code change ensures that the SortMergeResultPartitionReadScheduler will only request buffer after the BatchShuffleReadBufferPool is destoryed, which will trigger the handling failed logic and releaseSortMergeSubpartitionReader.

[reswqa]

Thanks for the quick review!