Core performance tests improvements (quick wins)

[swissspidy]

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.

The following items have been discussed with @felixarntz:

Short term:

• Update baseline in tests to 6.7
• Test TT5 theme
• Raise awareness of the dashboard among the team b adding to the handbook and any checklists, so people check it more regularly

Mid term:

• 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)
• With such improvements in place, ensure that all possible data is shown in the dashboard (e.g. data for all themes can be viewed)
• Improve dummy content to be more realistic and document it
• Test single post URLs as well.

Bonus:

• We didn't send data to the dashboard for a while (May - October). During this time, and probably after the 6.6 release (August), there was a TTFB regression in block themes. Find commit in trunk between August and October to find potential culprit.
• Build some alerting if nothing gets reported or if there is a drastic change in metrics

[joemcgill]

Test TT5 theme

There is https://core.trac.wordpress.org/ticket/62148 open to track this, along with this PR that needs to be reviewed.