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: #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

Author: Simek

packages/gradle-plugin/package.json

@@ -31,6 +31,7 @@
     "gradlew.bat",
     "README.md",
     "react-native-gradle-plugin",
+    "!react-native-gradle-plugin/src/test",
     "settings-plugin",
     "shared",
     "shared-testutil"