add test

test/css/samples/at-layer/expected.css

@@ -0,0 +1 @@
+@layer base, special;@layer special{div.svelte-xyz{color:rebeccapurple}}@layer base{div.svelte-xyz{color:green}}

test/css/samples/at-layer/input.svelte

@@ -0,0 +1,17 @@
+<div>hello</div>
+
+<style>
+	@layer base, special;
+
+	@layer special {
+		div {
+			color: rebeccapurple;
+		}
+	}
+
+	@layer base {
+		div {
+			color: green;
+		}
+	}
+</style>