doc: add test_runner to subsystem

PR-URL: nodejs#52774
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz.nizipli@sentry.io>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>

doc/contributing/collaborator-guide.md

@@ -847,6 +847,7 @@ might impact an LTS release.
 | `lib/net`                                                                  | @bnoordhuis, @indutny, @nodejs/streams                                        |
 | `lib/repl`                                                                 | @nodejs/repl                                                                  |
 | `lib/{_}stream{*}`                                                         | @nodejs/streams                                                               |
+| `lib/internal/test_runner`                                                 | @nodejs/test\_runner                                                          |
 | `lib/timers`                                                               | @nodejs/timers                                                                |
 | `lib/util`                                                                 | @nodejs/util                                                                  |
 | `lib/zlib`                                                                 | @nodejs/zlib                                                                  |