make it possible to use stage0 libtest on compiletest With https://github.com/rust-lang/rust/pull/119899, building the library tree will require a stage 1 compiler. This is because `compiletest` is defined as a `ToolStd` (since https://github.com/rust-lang/rust/pull/68019) in order to use the in-tree library. As a result, https://github.com/rust-lang/rust/pull/119899 makes certain development workflows more difficult as changes on the compiler tree will now require recompiling `compiletest` each time. This PR allows switching `ToolStd` to `ToolBootstrap` with a simple boolean option in `bootstrap.toml` to allow `compiletest` to use the stage 0 `libtest` instead. The changes under `src/ci` are clearly intended to make sure that `compiletest` doesn't break during future bootstrap beta bumps.