Add Unit Tests for CLI Argument Parsing

Set up unit tests (with [Jest](https://jestjs.io/) or similar) to verify that CLI arguments are parsed correctly.

**How to do it:**

* Add Jest to dev dependencies:

```bash
npm install --save-dev jest
```
* Create `src/cli.test.js` and write tests for your argument parsing logic.

Start by locating the CLI argument-parsing logic that `src/cli.test.js` should cover, then review the Jest setup and dev-dependency configuration. The work is done when Jest is available and `src/cli.test.js` verifies that CLI arguments are parsed correctly.

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