Remove JSEventLoopWatchdog from react-native package (#51614)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/51614

`JSEventLoopWatchdog` is not used in react-native package. This diff moves it to rn-tester which previously deep imported it from react-native (which we want to avoid).

Changelog:
[Internal]

Reviewed By: huntie

Differential Revision: D75410548

fbshipit-source-id: d4996742578e3b068e7acad9479394388b1907ac