Auto merge of #57842 - gnzlbg:extract_libtest, r=gnzlbg
Move libtest out of rust-lang/rust
This is a first step towards a number of goals explained in this internals post: https://internals.rust-lang.org/t/a-path-forward-towards-re-usable-libtest-functionality-custom-test-frameworks-and-a-stable-bench-macro
This PR does not fully remove libtest from rust-lang/rust, we keep a shim that imports and re-exports the external libtest, and adds the proc_macro dependency, etc.
r? @alexcrichton
cc @djrenren @petrochenkov