Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/33990
This diff moves the setting of some CPP flags from the main React native pods file to a dedicated file.
It also introduces some tests and it improves the Test Mocks we have
## Changelog
[iOS][Changed] - Move the `modify_flags_for_new_architecture` method to separate ruby file
Reviewed By: cortinico
Differential Revision: D37040927
fbshipit-source-id: 037ddaf123d01f3a2fd622b8a0cd10535da70b92