Update ui tests to `ui_test` crate

[xldenis]

Oil from the Rust team has made the ui_test crate, which is an extracted version of Rustc's compiletest crate, it makes it easy to set up and diff ui tests like the ones in should_succeed and even has GitHub action integration to report errors inline in PRs! I've set it up for Gillian-Rust, and I think we should do so as well here.