add regression test for missing class on elem with bind and spread (#3668)

relates to #2707

test/runtime/samples/class-with-spread-and-bind/_config.js

@@ -0,0 +1,18 @@
+export default {
+	props: {
+		primary: true,
+	},
+
+	html: `<div class="test-class primary" role="button"></div>`,
+
+	test({ assert, component, target, window }) {
+		component.primary = true;
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<div class="test-class primary" role="button"></div>
+		`
+		);
+	},
+};

test/runtime/samples/class-with-spread-and-bind/main.svelte

@@ -0,0 +1,11 @@
+<script>	
+	let primary = true;	
+	let elem;
+</script>
+
+<div
+	bind:this={elem}
+	class="test-class"
+	class:primary
+  {...{ role: 'button' }} 
+></div>