Add test case for @const async function with await bug

Co-authored-by: dummdidumm <[email protected]>

Author: Copilot

packages/svelte/tests/runtime-runes/samples/const-async-function-await/_config.js

@@ -0,0 +1,31 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<nav><ul><button>Tab 1</button><button>Tab 2</button><button>Tab 3</button></ul></nav> <h1>Tab 1</h1>`,
+
+	async test({ assert, target }) {
+		const [btn1, btn2, btn3] = target.querySelectorAll('button');
+
+		btn2.click();
+		flushSync();
+		assert.htmlEqual(
+			target.innerHTML,
+			`<nav><ul><button>Tab 1</button><button>Tab 2</button><button>Tab 3</button></ul></nav> <h1>Tab 2</h1>`
+		);
+
+		btn3.click();
+		flushSync();
+		assert.htmlEqual(
+			target.innerHTML,
+			`<nav><ul><button>Tab 1</button><button>Tab 2</button><button>Tab 3</button></ul></nav> <h1>Tab 3</h1>`
+		);
+
+		btn1.click();
+		flushSync();
+		assert.htmlEqual(
+			target.innerHTML,
+			`<nav><ul><button>Tab 1</button><button>Tab 2</button><button>Tab 3</button></ul></nav> <h1>Tab 1</h1>`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/const-async-function-await/main.svelte

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+<script>
+	const tabs = [
+		{ id: 'tab1', label: 'Tab 1'},
+		{ id: 'tab2', label: 'Tab 2' },
+		{ id: 'tab3', label: 'Tab 3' }
+	]
+	
+	let activeTab = $state('tab1')
+
+	const doSomething = async () => {
+		console.log('doSomething called')
+	}
+</script>
+
+<nav>
+	<ul>
+		{#each tabs as tab (tab.id)}
+			{@const switchTab = async () => {
+				console.log('switchTab called', tab.id)
+				activeTab = tab.id
+				await doSomething()
+			}}
+			
+			<button onclick={switchTab}>
+				{tab.label}
+			</button>
+		{/each}
+	</ul>
+</nav>
+
+{#if activeTab == 'tab1'}
+	<h1>Tab 1</h1>
+{:else if activeTab == 'tab2'}
+	<h1>Tab 2</h1>
+{:else if activeTab == 'tab3'}
+	<h1>Tab 3</h1>
+{/if}