This class originated in the very first commit of `rustdoc_ng`, and was used
to add a color border around the item decl based on its kind.
https://github.com/rust-lang/rust/blob/6ebf4c340e30786bb225442bfd3c6440dc83e0a6/src/rustdoc_ng/html/static/main.css#L102-L106
The item decl no longer has a border, and there aren't any
kind-specific styles in modern rustdoc's rendering of this UI item.
Most of this commit is updating test cases so that they use `item-decl` to
find the `<pre>` tag instead of relying on the fact that the class name
had `rust {kind}` in it while other `<pre>` tags only had class `rust`.