Move async/await UI tests into a folder

[cramertj]

We want to move all async / await tests (run-pass, ui, ...) into a single folder under src/test/ui/async-await to facilitate overview and the eventual stabilization report. Organization beyond that into sub-folders would also be useful to further make everything clear and reviewable.

[Centril]

Labeling as "blocking" since this should be done as part of the run up to the report.

[varkor]

Fixed in #61138.