tests: runtime: trap exceptions in event-handler-dynamic

Fail if initial button click causes an exception.

test/runtime/samples/event-handler-dynamic/_config.js

@@ -14,8 +14,14 @@ export default {
 		);
 
 		const event = new window.MouseEvent('click');
+		let err = "";
+		window.addEventListener('error', (e) => {
+			e.preventDefault();
+			err = e.message;
+		});
 
 		await button.dispatchEvent(event);
+		assert.equal(err, "", err);
 		assert.htmlEqual(target.innerHTML, `
 			<p>
 				<button>set handler 1</button>