feat: Test it

packages/svelte/tests/runtime-runes/samples/snippet-complicated-defaults/_config.js

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+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		/** @type {HTMLButtonElement | null} */
+		const increment = target.querySelector('#increment');
+		/** @type {HTMLButtonElement | null} */
+		const change_ref = target.querySelector('#change-ref');
+		/** @type {HTMLParagraphElement | null} */
+		const count = target.querySelector('#count');
+		/** @type {HTMLParagraphElement | null} */
+		const fallback_count = target.querySelector('#fallback-count');
+
+		assert.htmlEqual(count?.innerHTML ?? '', 'Count: 0');
+		assert.htmlEqual(fallback_count?.innerHTML ?? '', 'Fallback count: 0');
+
+		await increment?.click();
+		assert.htmlEqual(count?.innerHTML ?? '', 'Count: 1');
+		assert.htmlEqual(fallback_count?.innerHTML ?? '', 'Fallback count: 0');
+
+		await change_ref?.click();
+		await increment?.click();
+		assert.htmlEqual(count?.innerHTML ?? '', 'Count: 1');
+		assert.htmlEqual(fallback_count?.innerHTML ?? '', 'Fallback count: 1');
+
+		await change_ref?.click();
+		await increment?.click();
+		assert.htmlEqual(count?.innerHTML ?? '', 'Count: 2');
+		assert.htmlEqual(fallback_count?.innerHTML ?? '', 'Fallback count: 1');
+	}
+});

packages/svelte/tests/runtime-runes/samples/snippet-complicated-defaults/main.svelte

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+<script>
+	function box(value) {
+		let state = $state(value);
+		return {
+			get value() {
+				return state
+			},
+			set value(v) {
+				state = v
+			}
+		}
+	}
+
+  let count = box(0);
+	let fallback_count = box(0);
+	let toggle_state = $state(false);
+</script>
+
+{#snippet counter(c = count)}
+	<p id="count">Count: {count.value}</p>
+	<p id="fallback-count">Fallback count: {fallback_count.value}</p>
+  <button id="increment" on:click={() => (c.value += 1)}>Click to change referenced state value</button>
+	<button id="change-ref" on:click={() => toggle_state = !toggle_state}>Click to change state reference</button>
+{/snippet}
+
+{@render counter(toggle_state ? fallback_count : undefined)}