test(e2e-nextjs): Check NextJS client error transaction field (#11424)

Super small PR to check that an error event has a valid `transaction`
field in NextJS.
Besides testing, there was nothing specifically to do for NextJS since
as far as I can tell we never update a pageload or navigation span name.

ref #10846

Author: Lms24

dev-packages/e2e-tests/test-applications/nextjs-app-dir/tests/exceptions.test.ts

@@ -19,6 +19,8 @@ test('Sends a client-side exception to Sentry', async ({ page }) => {
   const errorEvent = await errorEventPromise;
   const exceptionEventId = errorEvent.event_id;
 
+  expect(errorEvent.transaction).toBe('/');
+
   await expect
     .poll(
       async () => {