test: enable test-debugger-pid

Now that `node debug` is an alias for `node inspect`, it's possible that
`node-debug-pid` can run reliably. Modify for current behavior and move
from `disabled` to `parallel`.

PR-URL: https://github.com/nodejs/node/pull/12770


Reviewed-By: Jan Krems <jan.krems@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>