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`
7dd170fccb