Rollup merge of #144813 - jieyouxu:no-top-level-tests, r=Kobzol

Add a tidy check to prevent adding UI tests directly under `tests/ui/`

This PR implements rust-lang/compiler-team#902.

Only the last commit (adding the new check) is functional; earlier commits are just small drive-by changes to make the other ui/ui-fulldeps checks more logically contained.

r? ```@Kobzol``` (or compiler)

Author: samueltardieu