[TASK][EASY] Add tests for Spark saveToFile function

[cxzl25]

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EASY

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Skill requirements

Familiar with Kyuubi and Spark.

Background and Goals

Currently the Spark saveToFile feature enabled via kyuubi.operation.result.saveToFile.enabled lacks testing, and modifying this related feature can easily lead to regressions.

Implementation steps

Add test

Additional context

Introduction of 2024H1 Kyuubi Code Contribution Program

[lsm1]

please assign to me