Guidelines for locale-sensitive testing in test262

[ptomato]

Slides: https://ptomato.name/talks/tc39tg2-2024-10
Notes from discussion: https://docs.google.com/document/d/1LktHEvOixPhtVynhYt_f944YVvlUy-pyuxrVhbXzTpU/edit?tab=t.0#heading=h.od036whynwor

In the 2024-10-24 meeting I presented the above slide deck, asking for feedback on best practices for testing Intl APIs in test262. Let's open this issue for further discussion and feedback.

[longlho]

Dropping a quick thought: I generally agree that test262 should contain more output matching tests. However, whenever I have to upgrade cldr for formatjs most of our tests also need updated, even for popular locales. Output matching is highly dependent on data source and that's also not in the spec :(

[ben-allen]

Thought: would it be worthwhile/appropriate to include a refined version of these guidelines in ecma402's docs directory? @ptomato @gibson042