Core performance tests improvements (2025 edition)

We talked about potential improvements to the core performance test suite in the past, but haven't worked on it much so far.

After we saw some slight performance regressions after 6.7 not only in lab data but also in the field, now is a good time to at least work on some quick wins.

[!NOTE]
This is split out from #1093, which is a larger ticket with long discussions about multiple aspects of performance testing.

Q1:

• Update baseline in tests to 6.7
• Test TT5 theme
• Contribute improvements to the codevitals repo to make it less overwhelming when there are more stats (e.g. a checkbox to hide/show certain fields)
• Test single post URLs as well.

Q2:

• Raise awareness of the dashboard among the team by adding to the handbook and any checklists
• Emulate broadband network conditions in core performance tests
  • Requires https://github.com/WordPress/gutenberg/pull/69865

Q3:

• Improve dummy content to be more realistic and document it

Stretch goals:

• Build some alerting if nothing gets reported or if there is a drastic change in metrics

Start by reviewing the core performance test suite and the linked Gutenberg pull request for broadband network emulation. Choose a specific unchecked item, such as handbook awareness, realistic dummy content, or alerting, and confirm its scope with the existing discussion. Done means the selected improvement is implemented, documented where requested, and verified by the relevant performance tests or reporting.