test: improve variable names in pty_helper.py

Using names like `parent_fd` and `child_fd` is more accurate here,
and doesn’t come with unnecessary negative connotations, even if
the previous naming is somewhat common terminology here.

PR-URL: https://github.com/nodejs/node/pull/28688


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>