Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/43053

- Relocate under `scripts/e2e/` (also move util used only by this cript).
- Type as Flow (to catch trivial errors). Some cleanup of `log()` calls as errors.

Changelog: [Internal]

Reviewed By: lunaleaps

Differential Revision: D53813023

fbshipit-source-id: 05caf415ec0bf3739a6f7fec3afd385a195f42e9