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

.github/actions/test-ios-helloworld/action.yml

@@ -83,7 +83,13 @@ runs:
 
         yarn bootstrap ios "${args[@]}" | cat
 
-    - name: Build HelloWorld  project
+    - name: Run Helloworld tests
+      shell: bash
+      run: |
+        cd packages/helloworld
+        yarn test
+
+    - name: Build HelloWorld project
       shell: bash
       run: |
         cd packages/helloworld