Fix output: standalone test for app directory (#43618)
This test was ensuring `app` directory is working correctly with
`output: 'standalone'`. Instead of removing config we should
conditionally set it for the test.
Reverts vercel/next.js#43607
x-ref:
https://github.com/vercel/next.js/actions/runs/3594732606/jobs/6055832699
x-ref:
https://github.com/vercel/next.js/actions/runs/3594732606/jobs/6055832421
