Add test case to MaskMergedResult

[FrostyHec]

Fixes #30591

Changes proposed in this pull #request:

Add test case to MaskMergedResult

Additional: Sorry for bothering, this is my first time trying to participate in this project, so I may have some issues with the project's coding standards: In this test unit, using BeforeEach might improve code clarity and reduce code duplication, but this might require introducing the usage of "lenient", which some projects might not allow. May I ask if our project allows lenient? Thanks~

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