Rollup merge of #139386 - onur-ozkan:configurable-compiletest-libtest, r=jieyouxu,kobzol

make it possible to use stage0 libtest on compiletest

With rust-lang/rust#119899, building the library tree will require a stage 1 compiler. This is because `compiletest` is defined as a `ToolStd` (since rust-lang/rust#68019) in order to use the in-tree library. As a result, rust-lang/rust#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.

Author: Zalathar