[lib] Introduce basic tests for SET_MESSAGE_STORE_MESSAGES action

Summary:
Had trouble debugging D4508 because I couldn't "attach" debugger to hermes in time for message-reducer.js:SET_MESSAGE_STORE_MESSAGES to hit.

Ended up testing via console.log(...) and pasting values into a text file to ensure that things were as expected. The stuff I jotted down lent itself well to writing basic unit tests so figured I might as well put it up.

Depends on D4508

Test Plan: Ran tests

Reviewers: jacek, marcin, tomek

Reviewed By: tomek

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4560