Add jest tests

[github-learning-lab[bot]]

What is this pull request doing?

This pull request introduces Jest, a popular JavaScript testing framework. We'll use it to learn how to use it for continuous integration.

When the bot asks you to, merge this pull request.

[github-learning-lab[bot]]

Great job! Go ahead and merge this pull request so your changes are automatically shared with anyone that contributes to this project.

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[github-learning-lab[bot]]

Let's go to the next step.