Rollup merge of #146929 - Zalathar:capture, r=Kobzol,jieyouxu

compiletest: Remove old-output-capture and become a stage0 bootstrap tool

The new compiletest output-capture system became the default in rust-lang/rust#146574, and no problems have been reported since.

This PR therefore removes the old output-capture implementation from compiletest, and adjusts bootstrap to always build and test compiletest as a stage0 bootstrap tool.

In other words, compiletest no longer relies on any unstable features (such as `libtest` or `internal_output_capture`), and is now written entirely in stable Rust!

The compiletest self-tests still need access to an in-tree rustc, in order to obtain information via `rustc --print`, so we interpret `--stage` as indicating the stage of that secondary compiler, but always use the stage0 compiler to build compiletest itself.

r? Kobzol