Add test for reexported hidden item with `--document-hidden-items` Coming from [this discussion on zulip](https://rust-lang.zulipchat.com/#narrow/stream/266220-t-rustdoc/topic/Using.20cargo-semver-checks.20in.20rustdoc.20JSON.20tests.3A.20revisited). cc ``@aDotInTheVoid`` r? ``@notriddle``