test: Set up Vitest

[dcalhoun]

What?

Set up an automated JavaScript testing environment.

Why?

Improve project stability. Close CMM-285.

How?

Leverage several dependencies already used by the Gutenberg project, with one divergence—use Vitest rather than Jest.

Attempts to set up an environment with vitest-jest and Gutenberg's jest-preset-default were unsuccessful, as I encountered numerous build errors related to Babel configuration. Using Jest, Vite, and jest-preset-default together may not straightforward. Also, vitest-jest appears to be an abandoned project. Vitest does have differences from Jest, but remains largely compatible.

In the future, we could reengage explorations for how we might further align with Gutenberg and use jest-preset-default.

Testing Instructions

N/A, no user-facing changes.

Accessibility Testing Instructions

N/A, no user-facing changes.

Screenshots or screencast

N/A, no user-facing changes.

[Copilot]

Pull Request Overview

This PR sets up an automated JavaScript testing environment using Vitest instead of Jest for the Gutenberg project. The key changes include adding Vitest configuration files (vitest.setup.js and vitest.config.js), updating test files (editor-load-notice.test.jsx) to work with Vitest, and modifying build scripts (package.json, Makefile, and Buildkite pipeline) and documentation to support JavaScript tests.

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
vitest.setup.js	Configures auto-cleanup for tests using Testing Library with Vitest.
vitest.config.js	Defines the Vitest configuration including plugins and setup files.
src/components/editor-load-notice.test.jsx	Adds tests for EditorLoadNotice covering multiple user interactions.
package.json	Updates scripts and dependencies to include Vitest and Testing Library.
mocks/@wordpress/i18n/index.js	Provides a simple mock for i18n functions using Vitest’s vi.fn().
mocks/@wordpress/components/index.jsx	Mocks a Notice component for testing purposes.
README.md	Enhances documentation with instructions for running JavaScript and other tests.
Makefile	Adds a target to run JavaScript tests via npm and Vitest.
.buildkite/pipeline.yml	Introduces a step to run JavaScript tests on the CI pipeline.