Wire up testing to ./x.py test in rust-lang/rust

[bjorn3]

• Build system: Allow storing the target dir in a specified location #1272
• Copy build_sysroot/ to download/ to avoid modifying the source directory (Avoid clobbering build_system/ and ~/.cargo/bin #1338)
• Remove hyperfine dependency for regular testing (Avoid clobbering build_system/ and ~/.cargo/bin #1338)
• Retry downloads on network failure #1280 (caacef2)
• Make repo download code robust against repo renames #1282
• Support offline builds using cargo fetch (Many build system improvements #1339)
• Support vendoring external dependencies using cargo vendor (Implement vendoring #1342 will be done in x.py instead)
• Disable abi-checker outside of cg_clif's CI. It writes to it's source dir and is rather slow. (7095783)
• Drop testing of simple-raytracer. Just keep it for benchmarking, but don't download it in ./y.rs prepare. (3ba9b13)
• Skip jit tests in rust's CI (134dc33)
• Support building with specified rustc and cargo rather than rustup managed one (Allow building and testing without rustup #1374)
• Transform async ResumeTy in generator transform rust#105977 (to fix cg_clif on newer nightlies, updated rustc in 6c58be8)
• Modify rustbuild (Run part of cg_clif's tests in CI rust#112701)

[jyn514]

Support building with specified rustc and cargo rather than rustup managed one

If you respect RUSTC and CARGO, I think bootstrap already sets those today :) I wouldn't expect you to need a new argument to y.rs or anything like that.

https://github.com/bjorn3/rustc_codegen_cranelift/issues/1282

cc https://rust-lang.zulipchat.com/#narrow/stream/238009-t-compiler.2Fmeetings/topic/.5Bweekly.5D.202023-01-05/near/319616098 - I think this is ok not to require for now :)

[bjorn3]

Opened rust-lang/rust#112701