RNTester enable concurrent root when using Fabric (#41166)

Summary:
RNTester's `AppDelegate` override `prepareInitialProps` method of super class `RCTAppDelegate` https://github.com/facebook/react-native/blob/70acd3f7d9edae9e40cc4603bede9778da281a85/packages/react-native/Libraries/AppDelegate/RCTAppDelegate.mm#L152, so we missed `concurrentRoot` initial prop.

![image](https://github.com/facebook/react-native/assets/5061845/12af5815-afe6-46f0-8107-54ca443b4962)

cc javache cipolleschi

## Changelog:

[IOS] [FIXED] - RNTester enable concurrent root when using Fabric

Pull Request resolved: https://github.com/facebook/react-native/pull/41166

Test Plan: Warning disappear.

Reviewed By: cipolleschi

Differential Revision: D50596693

Pulled By: javache

fbshipit-source-id: d73a17cd137b3088405f86b739cb0ed7b5a9839e