add await blowback reactive test case

test/runtime/samples/await-then-blowback-reactive/Widget.svelte

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+<script>
+  export let options = [];
+  export let index = 0;
+  export let value;
+
+  $: {
+		value = options[index];
+  }
+</script>
+
+<select bind:value={index}>
+  {#each options as option, i}
+    <option value={i}>{option}</option>
+  {/each}
+</select>

test/runtime/samples/await-then-blowback-reactive/_config.js

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+export default {
+	async test({ assert, component, target }) {
+		assert.htmlEqual(target.innerHTML, 'Loading...');
+
+		await component.promise;
+		const span = target.querySelector('span');
+		assert.equal(span.textContent, 'a');
+
+		const select = target.querySelector('select');
+		const options = [...target.querySelectorAll('option')];
+
+		const change = new window.Event('change');
+
+		options[1].selected = true;
+		await select.dispatchEvent(change);
+
+		assert.equal(span.textContent, 'b');
+	}
+};

test/runtime/samples/await-then-blowback-reactive/main.svelte

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+<script>
+	import Widget from './Widget.svelte';
+	let promise = Promise.resolve(['a', 'b']);
+	let value;
+</script>
+
+{#await promise}
+	Loading...
+{:then options}
+	<Widget {options} bind:value />
+	<span>{value}</span>
+{/await}