chore: add tests for custom elements properties

Author: edoardocavazza

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

@@ -0,0 +1,15 @@
+import { test } from '../../test';
+
+export default test({
+	mode: ['client', 'server'],
+	async test({ assert, target }) {
+		const my_element = /** @type HTMLElement & { object: { test: true }; } */ (target.querySelector('my-element'));
+		const my_link = /** @type HTMLAnchorElement & { object: { test: true }; } */ (target.querySelector('a'));
+		assert.equal(my_element.getAttribute('string'), 'test');
+		assert.equal(my_element.hasAttribute('object'), false);
+		assert.deepEqual(my_element.object, { test: true });
+		assert.equal(my_link.getAttribute('string'), 'test');
+		assert.equal(my_link.hasAttribute('object'), false);
+		assert.deepEqual(my_link.object, { test: true });
+	}
+});

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

@@ -0,0 +1,2 @@
+<my-element string="test" object={{ test: true }}></my-element>
+<a is="my-link" string="test" object={{ test: true }}></a>