add a failing test

packages/svelte/tests/runtime-runes/samples/state-onchange-child/_config.js

@@ -0,0 +1,11 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs }) {
+		const btn = target.querySelector('button');
+
+		flushSync(() => btn?.click());
+		assert.deepEqual(logs, ['b changed']);
+	}
+});

packages/svelte/tests/runtime-runes/samples/state-onchange-child/main.svelte

@@ -0,0 +1,13 @@
+<script>
+	let a = $state({});
+
+	let b = $state({ count: 0 }, {
+		onchange() {
+			console.log('b changed');
+		}
+	});
+
+	a.b = b;
+</script>
+
+<button onclick={()=> b.count++}>{b.count}</button>