test: add test case for plugin bootstrap (#3529)

tests/kit/utils/load-page.ts

@@ -5,3 +5,7 @@ export const webUrl = 'http://localhost:8080';
 export async function openHomePage(page: Page) {
   await page.goto(webUrl);
 }
+
+export async function openPluginPage(page: Page) {
+  await page.goto(`${webUrl}/_plugin/index.html`);
+}