Update iOS RNTester build artifacts in .gitignore (#51579)

Summary:
Running `yarn prepare-ios` creates some build artifacts folders, which can be added to the .gitignore as they are generated.

## Changelog:

[INTERNAL] - Update iOS RNTester build artifacts in .gitignore

Pull Request resolved: https://github.com/facebook/react-native/pull/51579

Test Plan: N/A

Reviewed By: cortinico

Differential Revision: D75389176

Pulled By: cipolleschi

fbshipit-source-id: ecb59051f5bb25b6be8fca9ac2ebf408adc5c7b7