add test from gh-2086

Author: Rich-Harris

test/runtime/samples/head-detached-in-dynamic-component/A.svelte

@@ -0,0 +1,5 @@
+<svelte:head>
+	<meta name="description" content="A"/>
+</svelte:head>
+
+A

test/runtime/samples/head-detached-in-dynamic-component/B.svelte

@@ -0,0 +1,5 @@
+<svelte:head>
+	<meta name="description" content="B"/>
+</svelte:head>
+
+B

test/runtime/samples/head-detached-in-dynamic-component/_config.js

@@ -0,0 +1,15 @@
+export default {
+	html: `
+		A
+	`,
+
+	test({ assert, component, window }) {
+		component.x = false;
+
+		const meta = window.document.querySelectorAll('meta');
+
+		assert.equal(meta.length, 1);
+		assert.equal(meta[0].name, 'description');
+		assert.equal(meta[0].content, 'B');
+	}
+};

test/runtime/samples/head-detached-in-dynamic-component/main.svelte

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+<script>
+	import A from './A.svelte';
+	import B from './B.svelte';
+
+	export let x = true;
+</script>
+
+<svelte:component this="{x ? A : B}"/>