Do not include test files in `gradle-plugin` bundle (#56193)

Summary:
While analysing dependency trees of several React Native libraries I have spotted that `react-native/gradle-plugin` package bundle includes test files.

This PR adds the exclude rule in package.json `files` array which prevents from adding test files to the dist.

## Changelog:

[INTERNAL] [CHANGED] - Do not include test files in `react-native/gradle-plugin` package bundle

Pull Request resolved: https://github.com/facebook/react-native/pull/56193

Test Plan:
```
npm pack --dry-run
```

### Before

<img width="1026" height="302" alt="Screenshot 2026-03-23 at 13 46 43" src="https://github.com/user-attachments/assets/22e0458f-b4ee-4ffe-8c5c-21204be1f983" />

### After

<img width="1026" height="302" alt="Screenshot 2026-03-23 at 13 46 59" src="https://github.com/user-attachments/assets/9c558a5b-3720-424b-9f6f-bb79048454bb" />

Reviewed By: christophpurrer

Differential Revision: D97753123

Pulled By: cortinico

fbshipit-source-id: 98b614115fca56b9abcbbd54109053689805b175