Auto merge of #124345 - Urgau:compiletest-check-cfg, r=jieyouxu

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`