doc: update "Testing and CI" in Collaborator Guide

Update "Testing and CI" in the Collaborator Guide. Remove redundant
material. Shorten sentences. Remove incorrect material. (Specifically,
we don't require test cases to be included in all pull requests that
modify exectuable code. For example, if code is refactored, then passing
existing tests is sufficient.)

PR-URL: https://github.com/nodejs/node/pull/24884


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>