test: add CSS tests for class directive compound selectors

packages/svelte/tests/css/samples/class-directive-compound-selectors/_config.js

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+import { test } from '../../test';
+
+export default test({
+	warnings: [
+		{
+			code: 'css_unused_selector',
+			message: 'Unused CSS selector ".unused"\nhttps://svelte.dev/e/css_unused_selector',
+			start: {
+				line: 18,
+				column: 1,
+				character: 231
+			},
+			end: {
+				line: 18,
+				column: 8,
+				character: 238
+			}
+		}
+	]
+});

packages/svelte/tests/css/samples/class-directive-compound-selectors/expected.css

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+.sidebar.collapsed.svelte-xyz {
+		width: 60px;
+	}
+	
+	.collapsed.svelte-xyz .content:where(.svelte-xyz) {
+		opacity: 0;
+	}
+	
+	/* (unused) .unused {
+		color: red;
+	}*/

packages/svelte/tests/css/samples/class-directive-compound-selectors/input.svelte

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+<script>
+	let collapsed = false;
+</script>
+
+<div class="sidebar" class:collapsed={collapsed}>
+	<div class="content">Content</div>
+</div>
+
+<style>
+	.sidebar.collapsed {
+		width: 60px;
+	}
+	
+	.collapsed .content {
+		opacity: 0;
+	}
+	
+	.unused {
+		color: red;
+	}
+</style>