Ship compiletest as part of rustc-dev

[Manishearth]

(Maybe we should have a meta tracking issue for subtree things, e.g. moving rustfmt and rustdoc over to subtree)

As clippy is fleshing out the subtree model, I wonder if it's worth shipping compiletest in rustc-dev so that rustc and clippy are always using the same compiletest.

cc @Mark-Simulacrum @flip1995

[RalfJung]

Miri would also be interested in this (though porting it entirely to ui tests could be hard in case compiletest lost support for some of the other modes).

[euclio]

This is a great idea! Notably, clippy's compiletest doesn't currently have #59408 in it, which causes test output that has backslashes to be pretty confusing: https://github.com/rust-lang/rust-clippy/blob/742706511c9f33c6a0d4380392e513e5249057e3/tests/ui/print_with_newline.stderr#L4

[Mark-Simulacrum]

Shipping the binary seems feasible, though it would be a small bump in CI times.

[tamird]

This is probably no longer relevant now that clippy is using ui-test?

[RalfJung]

Yeah, and Miri too.