Add a basic Jest test to helloworld (#51534)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/51534

Aims to give us CI coverage of React Native's Jest preset, preventing future bugs like https://github.com/facebook/react-native/pull/51525.

Changes:
- Add a basic "it renders" Jest test to helloworld
- Add "Run Helloworld tests" step to `test-ios-helloworld` job in CI
- Also convert helloworld's `App.tsx` to TypeScript, as easiest way to unblock Jest JSX behaviour.

Changelog: [Internal]

Reviewed By: cortinico

Differential Revision: D75218901

fbshipit-source-id: 601155c59c4483696971df4c29d51549d97f49f2