Add testID to NewAppScreen Header Component (facebook#31652)

Summary:
The RN OSS release process includes manual testing that a new template app can be started under various platforms, JS engines, etc. This should ideally be automated, to help reduce wasted engineer-time, and to allow reliably increasing release velocity.

`react-native-windows` does already have tests to create and build template projects across our matrix, but they do not do any runtime validation on the newly created app. Adding a `testID` to the new app screen header gives us something to search for in black-box testing to validate that the app started successfully. This should help catch cases where a sample project in repo has changes not reflected in a newly created template app.

## Changelog

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[Internal] [Added] - Add testID to NewAppScreen Header Component

Pull Request resolved: facebook#31652

Test Plan: Did not manually validate the change, though did check that `ImageBackground` forwards props to `Image`, and that `Image` is aware of `testID` and will forward to the native component.

Reviewed By: kacieb

Differential Revision: D28907197

Pulled By: p-sun

fbshipit-source-id: db3974294afba25878383f1955cad37b69d95da3

Author: NickGerleman

Libraries/NewAppScreen/components/Header.js

@@ -19,6 +19,7 @@ const Header = (): Node => {
   return (
     <ImageBackground
       accessibilityRole="image"
+      testID="new-app-screen-header"
       source={require('./logo.png')}
       style={[
         styles.background,