chore: add "untracked allows writes" test (#17930)

While we don't officially document it, `untrack` also allows to opt out
of the "unsafe mutation" validation, which is what we test here.

For anyone coming across this: USE WITH CAUTION. This can cause graph
inconsistencies leading to wrong values on initial render

Doing this because we're gonna make use of it ourselves for remote
functions, and this ensures we don't accidentally regress.

packages/svelte/tests/runtime-runes/samples/untrack-allows-writes/_config.js

@@ -0,0 +1,19 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+// While we don't officially document it, `untrack` also allows to opt out of the "unsafe mutation" validation, which is what we test here
+export default test({
+	html: '<button>0 0 0</button>',
+	test({ assert, target }) {
+		const button = target.querySelector('button');
+
+		flushSync(() => button?.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>1 1 2</button>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/untrack-allows-writes/main.svelte

@@ -0,0 +1,14 @@
+<script>
+	import { untrack } from "svelte";
+
+	let count = $state(0);
+	let mirrored = $state(0);
+	let double = $derived.by(() => {
+		untrack(() => {
+			mirrored = count;
+		});
+		return count * 2;
+	})
+</script>
+
+<button onclick={() => count++}>{count} {mirrored} {double}</button>