chore: add better test

Author: paoloricciuti

packages/svelte/tests/runtime-runes/samples/const-tag-boundary/_config.js

@@ -2,7 +2,16 @@ import { flushSync } from 'svelte';
 import { test } from '../../test';
 
 export default test({
-	html: '<button></button> 2',
-	mode: ['client']
-	// TODO fix reactivity lost in failed snippet and add a test here
+	html: '<button></button><p>2</p>',
+	mode: ['client'],
+	test({ target, assert }) {
+		const btn = target.querySelector('button');
+		const p = target.querySelector('p');
+
+		flushSync(() => {
+			btn?.click();
+		});
+
+		assert.equal(p?.innerHTML, '4');
+	}
 });

packages/svelte/tests/runtime-runes/samples/const-tag-boundary/main.svelte

@@ -8,7 +8,7 @@
 <svelte:boundary>
 	{@const double = test * 2}
 	{#snippet failed()}
-		{double}
+		<p>{double}</p>
 	{/snippet}
 	<FlakyComponent />
 </svelte:boundary>