add test

packages/svelte/tests/runtime-runes/samples/async-reactivity-loss-for-await/_config.js

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+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+
+	html: `<button>a</button><button>b</button><p>pending</p>`,
+
+	async test({ assert, target, warnings }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<button>a</button><button>b</button><h1>3</h1>');
+
+		assert.equal(
+			warnings[0],
+			'Detected reactivity loss when reading `values[1]`. This happens when state is read in an async function after an earlier `await`'
+		);
+
+		assert.equal(warnings[1].name, 'TracedAtError');
+
+		assert.equal(warnings.length, 2);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-reactivity-loss-for-await/main.svelte

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+<script>
+	let values = $state([1, 2]);
+
+	async function get_total() {
+		let total = 0;
+
+		for await (const n of values) {
+			total += n;
+		}
+
+		return total;
+	}
+</script>
+
+<button onclick={() => values[0]++}>a</button>
+<button onclick={() => values[1]++}>b</button>
+
+<svelte:boundary>
+	<h1>{await get_total()}</h1>
+
+	{#snippet pending()}
+		<p>pending</p>
+	{/snippet}
+</svelte:boundary>