I have read this document https://docs.appian.com/suite/help/25.2/Start_Rule_Tests_Applications_Smart_Service.html.
But I want to know is there any way to only test the unit test of a package?
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No way unfortunately. I would suggest you log a feature request with Appian Support to return unit test results when exporting a package via the API or the ability to run tests at the package level or even rule in the Smart Service.