add test for css variables

test/runtime/samples/inline-style-directive-css-vars/_config.js

@@ -0,0 +1,9 @@
+export default {
+	html: '<p style="--border-color: red;"></p>',
+
+	test({ assert, component, target, window }) {
+		component.myColor = 'blue';
+
+		assert.htmlEqual(target.innerHTML, '<p style="--border-color: blue;"></p>');
+	}
+};

test/runtime/samples/inline-style-directive-css-vars/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	export let myColor = "red";
+</script>
+
+<p style:--border-color={myColor} />