WIP - stable libtest

The migration of libtest to stable Rust is currently in progress.

You can find libtest at: https://github.com/rust-lang/libtest . If you need to make a change:

• perform the change there,
• do a new crates.io release, and
• send a PR to rust-lang/rust bumping the libtest version.

The roadmap of the migration is being tracked here: https://github.com/rust-lang/libtest/issues/2