Improve existing CI schema specs

What does this MR do and why?

Some small improvements that were not done in previous MRs:

• We created negative_tests/rules.yml to test for invalid workflow:rules:changes examples, but we did not include it in ci_schema_spec.js which runs all the tests for the CI schema 😅
• We can remove the MR links at the top of each file since our schema will continue to change and we will continue to add/modify these tests in the future. (The MR links were a note for myself in the past when I was combing through recent MRs with a schema change that did not have tests yet)
• As the CI schema grows, I think it's helpful to note what each config example is testing for in short comments.

Screenshots or screen recordings

N/A

How to set up and validate locally

yarn jest spec/frontend/editor/schema/ci/ci_schema_spec.js

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.