Add Jest test matchers for jsdom

[36degrees]

Include the @testing-library/jest-dom library which provides custom Jest matchers for asserting things about the state of the DOM, including things like what class an element has (toHaveClass), or the accessible description for an element (toHaveAccessibleDescription).

This as the first step in migrating our template tests from Cheerio to jsdom.

Part of #5010

[github-actions]

📋 Stats

File sizes

File	Size
dist/govuk-frontend-development.min.css	113.37 KiB
dist/govuk-frontend-development.min.js	42.34 KiB
packages/govuk-frontend/dist/govuk/all.bundle.js	88.16 KiB
packages/govuk-frontend/dist/govuk/all.bundle.mjs	82.83 KiB
packages/govuk-frontend/dist/govuk/all.mjs	981 B
packages/govuk-frontend/dist/govuk/govuk-frontend-component.mjs	359 B
packages/govuk-frontend/dist/govuk/govuk-frontend.min.css	113.36 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend.min.js	42.33 KiB
packages/govuk-frontend/dist/govuk/i18n.mjs	5.55 KiB
packages/govuk-frontend/dist/govuk/init.mjs	4.86 KiB

Modules

File	Size (bundled)	Size (minified)
all.mjs	78.45 KiB	40.31 KiB
accordion.mjs	22.71 KiB	12.85 KiB
button.mjs	5.98 KiB	2.69 KiB
character-count.mjs	22.4 KiB	9.92 KiB
checkboxes.mjs	5.83 KiB	2.83 KiB
error-summary.mjs	7.89 KiB	3.46 KiB
exit-this-page.mjs	17.1 KiB	9.26 KiB
header.mjs	4.46 KiB	2.6 KiB
notification-banner.mjs	6.26 KiB	2.62 KiB
password-input.mjs	15.15 KiB	7.25 KiB
radios.mjs	4.83 KiB	2.38 KiB
skip-link.mjs	4.39 KiB	2.18 KiB
tabs.mjs	10.13 KiB	6.11 KiB

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Action run for 182b60e

[romaricpascal]

nitpick Given the jest.config.mjs is at the root of the repository, I think those two should go in the root package.json. That's also where jest and its plugins are listed as dependencies. I think things work at the moment because Node hoists packages to the root node_modules when it installs dependencies from workspaces.

[romaricpascal]

Looking forward to use the new matchers. I think you need to update where the new dependencies have been installed, though 😊

[romaricpascal]

Neat! And nice find on the ESLint plugin as well ⛵