doc: add node-test-commit-custom-suites to docs

Add node-test-commit-custom-suites to the COLLABORATOR_GUIDE.

Refs: https://github.com/nodejs/node/pull/21909#issuecomment-406796234

PR-URL: https://github.com/nodejs/node/pull/21927


Reviewed-By: Lance Ball <lball@redhat.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>