Add testing framework to write test cases

After searching different testing framework, Jest seems to be the best choice so far as it supports mocking, asynchronous testing, parallel test execution in an easy way.

The issue proposes Jest but names no files, existing test setup, or test cases. Start by reviewing the repository's current scripts and testing configuration, then determine the initial test scope and how tests should be run. Done should be a defined, runnable testing setup that supports writing test cases.

Written by the indexing model from the issue text.