Add test case for set_custom_element_data on builtin custom elements

packages/svelte/tests/runtime-runes/samples/custom-element-attributes/_config.js

@@ -20,5 +20,8 @@ export default test({
 		const [value1, value2] = target.querySelectorAll('value-element');
 		assert.equal(value1.shadowRoot?.innerHTML, '<span>test</span>');
 		assert.equal(value2.shadowRoot?.innerHTML, '<span>test</span>');
+
+		const value_builtin = target.querySelectorAll('div');
+		assert.equal(value_builtin.shadowRoot?.innerHTML, '<span>test</span>');
 	}
 });