Add basic snapshot diffing mechanism (#55906)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55906

Changelog: [Internal]

Adds a simple snapshot diffing mode to the snapshot generation (`--check` argument).

Removes `RUN_ON_REACT_NATIVE` constant in favor of `--test` argument.

Reviewed By: cipolleschi

Differential Revision: D93407648

fbshipit-source-id: 7860bca27ba6784b9b8517ec86bd16b41af48d9b