Add shared-testutil folder to NPM files to be published (#45936)

Summary:
React-native 0.75 RC7 gradle sync is currently broken due to the fact that the `shared-testutil` folder is missing from the `react-native/gradle-plugin` npm package

## Changelog:

[INTERNAL] [ADDED] - Add shared-testutil folder to NPM files to be published

Pull Request resolved: https://github.com/facebook/react-native/pull/45936

Test Plan: N/A

Reviewed By: cipolleschi

Differential Revision: D60969631

Pulled By: cortinico

fbshipit-source-id: 850edfe0cf6b0e8174a1df9ea962d207d2ce0112