test: expand React subscribe test coverage (#3442)

The React-level subscribe.test.tsx only had a single test checking that
subscribe is a function. This adds comprehensive tests mirroring the
vanilla subscribe tests, covering state change notifications,
subscribeWithSelector integration, equality functions, unsubscription,
fireImmediately, and multiple subscriber behavior.

Co-authored-by: Danilo Britto <dbritto.dev@gmail.com>