chore: add async-merge-blocking-pending test from #18983 (#18989)

#18983 adds two new tests. One of them passes on `main`, so we can merge
it independently. The other only passes on `pt-1`, so I'll add it to
that branch

packages/svelte/tests/runtime-runes/samples/async-merge-blocking-pending/_config.js

@@ -0,0 +1,34 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons = '<button>x</button><button>toggle</button><button>pop</button>';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [x, toggle, pop] = target.querySelectorAll('button');
+
+		// X is deferred on its own await
+		x.click();
+		await tick();
+		// Y is deferred on the await inside the new branch
+		toggle.click();
+		await tick();
+		// Z hides that branch again, merges into Y, which merges into X
+		toggle.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<p>n = 0</p><p>x = 0</p>${buttons}`);
+
+		// the branch's await resolves while it is hidden
+		pop.click();
+		await tick();
+		// W shows the branch again and merges into X
+		toggle.click();
+		await tick();
+
+		// nothing is pending any more, so X commits
+		pop.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<p>n = 4</p><p>x = 1</p><p>cond: e</p>${buttons}`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-merge-blocking-pending/main.svelte

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+<script>
+	let n = $state(0);
+	let x = $state(0);
+	let cond = $state(false);
+	let pend = false;
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!pend) return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+<svelte:boundary>
+	<p>n = {n}</p>
+	<p>x = {await delay(x)}</p>
+	{#if cond}
+		<p>cond: {await delay('e')}</p>
+	{/if}
+
+	{#snippet pending()}
+		<p>loading</p>
+	{/snippet}
+</svelte:boundary>
+
+<button onclick={() => { pend = true; n++; x = 1; }}>x</button>
+<button onclick={() => { n++; cond = !cond; }}>toggle</button>
+<button onclick={() => deferred.pop()()}>pop</button>