rustdoc: Add GUI test for "Auto-hide item contents for large items" setting Part of https://github.com/rust-lang/rust/issues/66181. The `browser-ui-test` version update is because there wasn't `null` check for attributes so I added it (PR is [here](https://github.com/GuillaumeGomez/browser-UI-test/pull/440)). r? ``@notriddle``