Rename mis-named data providers discovered in #3421418

Do we have a coding standard for this? Seems something that we could use a sniff for.

@mstrelan, I'm not sure if there's a strict coding standard about data provider naming; I think it's more like a gradually applied best practice. But it is definitely worth considering a coding standard with a rule to go with it.

Tagging "Needs followup" for that, although it might also be worth checking the coding standards queue for an existing issue about it and/or postponing this on a coding standard rather than fixing it first.

#2918440: [policy, no patch] Discuss the standards for data providers phpunit based tests → is probably the best place to discuss it, postponing on that.

The other issue did not cover method naming in its scope; it was about requirements for parameter and return documentation. Also, it seems like there's an old, pre-Generator approach suggested that might no longer be relevant (?) so I'm not sure about postponing this on that.

Updated for the related issue @quietone separated out.