docs: fix testing docs (#18163)

extracted from https://github.com/sveltejs/svelte.dev/pull/1959

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

@@ -38,6 +38,21 @@ You can now write unit tests for code inside your `.js/.ts` files:
 
 ```js
 /// file: multiplier.svelte.test.js
+// @filename: multiplier.svelte.ts
+export function multiplier(initial: number, k: number) {
+	let count = $state(initial);
+
+	return {
+		get value() {
+			return count * k;
+		},
+		set: (c: number) => {
+			count = c;
+		}
+	};
+}
+// @filename: multiplier.svelte.test.js
+// ---cut---
 import { flushSync } from 'svelte';
 import { expect, test } from 'vitest';
 import { multiplier } from './multiplier.svelte.js';
@@ -80,6 +95,16 @@ Since Vitest processes your test files the same way as your source files, you ca
 
 ```js
 /// file: multiplier.svelte.test.js
+// @filename: multiplier.svelte.ts
+export function multiplier(getCount: () => number, k: number) {
+	return {
+		get value() {
+			return getCount() * k;
+		}
+	};
+}
+// @filename: multiplier.svelte.test.js
+// ---cut---
 import { flushSync } from 'svelte';
 import { expect, test } from 'vitest';
 import { multiplier } from './multiplier.svelte.js';
@@ -115,6 +140,10 @@ If the code being tested uses effects, you need to wrap the test inside `$effect
 
 ```js
 /// file: logger.svelte.test.js
+// @filename: logger.svelte.ts
+export function logger(fn: () => void) {}
+// @filename: logger.svelte.test.js
+// ---cut---
 import { flushSync } from 'svelte';
 import { expect, test } from 'vitest';
 import { logger } from './logger.svelte.js';
@@ -213,7 +242,7 @@ test('Component', () => {
 	expect(document.body.innerHTML).toBe('<button>0</button>');
 
 	// Click the button, then flush the changes so you can synchronously write expectations
-	document.body.querySelector('button').click();
+	document.body.querySelector('button')?.click();
 	flushSync();
 
 	expect(document.body.innerHTML).toBe('<button>1</button>');
@@ -226,6 +255,7 @@ test('Component', () => {
 While the process is very straightforward, it is also low level and somewhat brittle, as the precise structure of your component may change frequently. Tools like [@testing-library/svelte](https://testing-library.com/docs/svelte-testing-library/intro/) can help streamline your tests. The above test could be rewritten like this:
 
 ```js
+// @errors: 2339
 /// file: component.test.js
 import { render, screen } from '@testing-library/svelte';
 import userEvent from '@testing-library/user-event';