Add regression test for resize listener iframe z-index

test/runtime/samples/binding-width-height-z-index/_config.js

@@ -0,0 +1,7 @@
+export default {
+  async test({ assert, target }) {
+    const iframe = target.querySelector('iframe');
+
+    assert.equal(iframe.style.zIndex, '-1');
+  }
+};

test/runtime/samples/binding-width-height-z-index/main.svelte

@@ -0,0 +1,8 @@
+<script>
+	let offsetWidth = 0;
+	let offsetHeight = 0;
+</script>
+
+<div style="z-index: 42;" bind:offsetHeight bind:offsetWidth>
+	<h1>Hello</h1>
+</div>