PTFM-10582 "dx build" sets the developer list from dxapp.json.

* Improve messaging for adding PUBLIC to the authorized user list (when
  run non-interactively), which requires a similar user confirmation.

* The write_app_directory test helper can now be called more than once
  with the same directory to overwrite previously written app source.

* Document assumptions we were already relying on about the
  DXTEST_CREATE_APPS test flag.