Auto merge of #134738 - clubby789:forbid-output-ui, r=jieyouxu

compiletest: Support `forbid-output` in UI tests

The `forbid-output` directive is currently only run in incremental tests (although no incremental tests use it). There are some UI tests 'using' it, but it's doing nothing 😄 Let's fix this

Will also PR the dev guide to note this.

dev-guide PR: https://github.com/rust-lang/rustc-dev-guide/pull/2171