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: #45936

Test Plan: N/A

Reviewed By: cipolleschi

Differential Revision: D60969631

Pulled By: cortinico

fbshipit-source-id: 850edfe0cf6b0e8174a1df9ea962d207d2ce0112

packages/react-native-gradle-plugin/package.json

@@ -32,6 +32,7 @@
     "README.md",
     "react-native-gradle-plugin",
     "settings-plugin",
-    "shared"
+    "shared",
+    "shared-testutil"
   ]
 }