chore: add regression test for teardown value behavior (#18751)

#18746 would've introduced a bug that makes this test fail. Add it now
so we don't regress in the future

packages/svelte/tests/runtime-runes/samples/effect-loop-teardown-value/_config.js

@@ -0,0 +1,9 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target, logs }) {
+		flushSync(() => target.querySelector('button')?.click());
+		assert.deepEqual(logs, ['setup: two', 'cleanup: two']);
+	}
+});

packages/svelte/tests/runtime-runes/samples/effect-loop-teardown-value/main.svelte

@@ -0,0 +1,28 @@
+<script>
+	let start = $state(false);
+	let enabled = $state(false);
+	let advance = $state(false);
+	let value = $state('one');
+
+	$effect(() => {
+		if (start) {
+			value = 'two';
+			enabled = true;
+		}
+	});
+
+	$effect(() => {
+		if (enabled) {
+			console.log(`setup: ${value}`);
+			advance = true;
+
+			return () => console.log(`cleanup: ${value}`);
+		}
+	});
+
+	$effect(() => {
+		if (advance) enabled = false;
+	});
+</script>
+
+<button onclick={() => (start = true)}>start</button>