test: refactor test-debug-prompt

* Use cleaner `process.stdin.write('.exit')` to exit the process rather
  than `proc.kill()`.
* Move test to sequential. It uses the default port 9229. It will fail
  if another inspector test (or test using port 0) is already using that
  port. So it needs to be run sequentially rather than in parallel with
  other tests. (We haven't seen many failures with it yet because there
  aren't a lot of other inspector tests in parallel at this time.)

PR-URL: nodejs/node#15141
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>