test: move inspector test back to parallel, unmark flaky

After https://github.com/nodejs/node/pull/21182, the test is
no longer flaky.

Since it doesn’t use a static port, moving it to parallel
seems justified.

PR-URL: https://github.com/nodejs/node/pull/21806


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>