RN: Flowify `packages/rn-tester` (#51788)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/51788

Adds `flow` to the remaining files that are lacking it in the `packages/rn-tester` directory.

This also adds any necessary type annotations and fixes lint warnings.

Changelog:
[Internal]

Reviewed By: SamChou19815

Differential Revision: D75899307

fbshipit-source-id: 27a74ed0007b3b754446a45931c2c148312d5e3a