Add package.json script to run Fantom tests (facebook#48144)

Summary:
Pull Request resolved: facebook#48144

Changelog: [internal]

Adds package.json script to run Fantom tests:

```
yarn fantom
```

NOTE: At the moment, this only works on Meta's infra. We're working on making this available in OSS/Github CI.

Reviewed By: javache

Differential Revision: D66874962

fbshipit-source-id: d9746428b618a31ce0bf96c3233828cdba501dd6

package.json

@@ -31,6 +31,7 @@
     "test-typescript-offline": "dtslint --localTs node_modules/typescript/lib  packages/react-native/types",
     "test-typescript": "dtslint packages/react-native/types",
     "test": "jest",
+    "fantom": "JS_DIR='..' yarn jest --config packages/react-native-fantom/config/jest.config.js",
     "trigger-react-native-release": "node ./scripts/releases-local/trigger-react-native-release.js",
     "update-lock": "npx yarn-deduplicate"
   },