expand test to cover css vars

test/runtime/samples/inline-style-directive-string-variable-kebab-case/_config.js

@@ -1,6 +1,6 @@
 export default {
 	html: `
-		<div style="background-image: url(https://raw.githubusercontent.com/sveltejs/branding/master/svelte-vertical.png);"></div>
+		<div style="background-image: url(https://raw.githubusercontent.com/sveltejs/branding/master/svelte-vertical.png); --css-variable: rgba(0, 0, 0, 1);"></div>
 	`,
 
 	test({ assert, target, window }) {
@@ -8,10 +8,11 @@ export default {
 		const styles = window.getComputedStyle(div);
 
 		assert.equal(styles['background-image'], 'url(https://raw.githubusercontent.com/sveltejs/branding/master/svelte-vertical.png)');
+		assert.equal(styles.getPropertyValue('--css-variable'), 'rgba(0, 0, 0, 1)');
 
 		assert.htmlEqual(
 			target.innerHTML,
-			'<div style="background-image: url(https://raw.githubusercontent.com/sveltejs/branding/master/svelte-vertical.png);"></div>'
+			'<div style="background-image: url(https://raw.githubusercontent.com/sveltejs/branding/master/svelte-vertical.png); --css-variable: rgba(0, 0, 0, 1);"></div>'
 		);
 	}
 };

test/runtime/samples/inline-style-directive-string-variable-kebab-case/main.svelte

@@ -1,6 +1,11 @@
 <script>
 	let url =
 		"https://raw.githubusercontent.com/sveltejs/branding/master/svelte-vertical.png";
+
+	let alpha = 1;
 </script>
 
-<div style:background-image="url({url})" />
+<div
+	style:background-image="url({url})"
+	style:--css-variable="rgba(0, 0, 0, {alpha})"
+/>