docs: add testing with storybook (#16701)

* docs: add testing with storybook

* docs: remove video

* docs: address feedback

- consistent tabs vs. spaces in snippet
- add more Storybook details

* Revise Storybook testing documentation

condense, remove referer query parameters

* Update documentation/docs/07-misc/02-testing.md

Co-authored-by: Jeppe Reinhold <[email protected]>

* Tweaks

- Prose updates
- More helpful link

* make it its own section

---------

Co-authored-by: Simon H <[email protected]>
Co-authored-by: Jeppe Reinhold <[email protected]>

Author: 3 people

documentation/docs/07-misc/02-testing.md

@@ -160,9 +160,9 @@ export function logger(getValue) {
 
 ### Component testing
 
-It is possible to test your components in isolation using Vitest.
+It is possible to test your components in isolation, which allows you to render them in a browser (real or simulated), simulate behavior, and make assertions, without spinning up your whole app.
 
-> [!NOTE] Before writing component tests, think about whether you actually need to test the component, or if it's more about the logic _inside_ the component. If so, consider extracting out that logic to test it in isolation, without the overhead of a component
+> [!NOTE] Before writing component tests, think about whether you actually need to test the component, or if it's more about the logic _inside_ the component. If so, consider extracting out that logic to test it in isolation, without the overhead of a component.
 
 To get started, install jsdom (a library that shims DOM APIs):
 
@@ -246,6 +246,48 @@ test('Component', async () => {
 
 When writing component tests that involve two-way bindings, context or snippet props, it's best to create a wrapper component for your specific test and interact with that. `@testing-library/svelte` contains some [examples](https://testing-library.com/docs/svelte-testing-library/example).
 
+### Component testing with Storybook
+
+[Storybook](https://storybook.js.org) is a tool for developing and documenting UI components, and it can also be used to test your components. They're run with Vitest's browser mode, which renders your components in a real browser for the most realistic testing environment.
+
+To get started, first install Storybook ([using Svelte's CLI](/docs/cli/storybook)) in your project via `npx sv add storybook` and choose the recommended configuration that includes testing features. If you're already using Storybook, and for more information on Storybook's testing capabilities, follow the [Storybook testing docs](https://storybook.js.org/docs/writing-tests?renderer=svelte) to get started.
+
+You can create stories for component variations and test interactions with the [play function](https://storybook.js.org/docs/writing-tests/interaction-testing?renderer=svelte#writing-interaction-tests), which allows you to simulate behavior and make assertions using the Testing Library and Vitest APIs. Here's an example of two stories that can be tested, one that renders an empty LoginForm component and one that simulates a user filling out the form:
+
+```svelte
+/// file: LoginForm.stories.svelte
+<script module>
+	import { defineMeta } from '@storybook/addon-svelte-csf';
+	import { expect, fn } from 'storybook/test';
+
+	import LoginForm from './LoginForm.svelte';
+
+	const { Story } = defineMeta({
+		component: LoginForm,
+		args: {
+			// Pass a mock function to the `onSubmit` prop
+			onSubmit: fn(),
+		}
+	});
+</script>
+ 
+<Story name="Empty Form" />
+ 
+<Story
+	name="Filled Form"
+	play={async ({ args, canvas, userEvent }) => {
+		// Simulate a user filling out the form
+		await userEvent.type(canvas.getByTestId('email'), '[email protected]');
+		await userEvent.type(canvas.getByTestId('password'), 'a-random-password');
+		await userEvent.click(canvas.getByRole('button'));
+
+		// Run assertions
+		await expect(args.onSubmit).toHaveBeenCalledTimes(1);
+		await expect(canvas.getByText('You’re in!')).toBeInTheDocument();
+	}}
+/>
+```
+
 ## E2E tests using Playwright
 
 E2E (short for 'end to end') tests allow you to test your full application through the eyes of the user. This section uses [Playwright](https://playwright.dev/) as an example, but you can also use other solutions like [Cypress](https://www.cypress.io/) or [NightwatchJS](https://nightwatchjs.org/).