Docs: Update stand-alone test information

[tldahlgren]

This PR updates the stand-alone test documentation to improve the examples, emphasize points I find myself making during PR reviews, and provide tips that I think have been useful in my work with others.

• split test parts into separate section for emphasis and an includable link for PRs
• elaborate on and provide a more relevant example of building and running tests
• update, improve, and add examples of stand-alone tests (based on "common" PR feedback I've given)
• Add references (and links) to relevant sections to help people find the information they need
• update the test log's descriptions and status of tests to align with current outputs
• make better use of alternative admonitions for titles and or alternate colors

[alalazo]

Docs not building: https://readthedocs.org/projects/spack/builds/24752925/

[tldahlgren]

Docs not building: https://readthedocs.org/projects/spack/builds/24752925/

Had a useful error message too. Those literal end-strings...

[tldahlgren]

This PR is hung on CI but it is a docs-only change making that check irrelevant.

[alalazo]

I did a quick pass over docs, and it lgtm. Feel free to merge if you're not seeking further comments / opinions.