Enable `--check-cfg` by default in UI tests This PR enables-by-default `--check-cfg` in UI tests, now that it has become stable. To do so this PR does 2 main things: - it introduce the `no-auto-check-cfg` directive to `compiletest`, to prevent any `--check-cfg` args (only to be used for `--check-cfg` tests) - it updates the _remaining_[^1] UI tests by either: - allowing the lint when neither expecting the lint nor giving the check-cfg args make sense - give the appropriate check-cfg args - or expect the lint, when it useful [^1]: some preparation work was done in #123577 #123702 I highly recommend reviewing this PR commit-by-commit. r? `@jieyouxu`