Fixes [this issue](https://github.com/zeit/next.js/issues/4747)
I don't know what is the reason why the process does not finish, because it can be reproduced in this repo in many environments (my local mac os and Netlify pipeline).
However, it fixes the problem and it's 100% safe.
* Add build manifest
* Split out css since they don’t have exact name
* Remove pages map
* Fix locations test
* Re-run tests
* Get consistent open ports
* Fix static tests
* Add comment about Cache-Control header