rustbuild: Add support for crate tests + doctests

This commit adds support to rustbuild to run crate unit tests (those defined by
`#[test]`) as well as documentation tests. All tests are powered by `cargo test`
under the hood.

Each step requires the `libtest` library is built for that corresponding stage.
Ideally the `test` crate would be a dev-dependency, but for now it's just easier
to ensure that we sequence everything in the right order.

Currently no filtering is implemented, so there's not actually a method of
testing *only* libstd or *only* libcore, but rather entire swaths of crates are
tested all at once.

A few points of note here are:

* The `coretest` and `collectionstest` crates are just listed as `[[test]]`
  entires for `cargo test` to naturally pick up. This mean that `cargo test -p
  core` actually runs all the tests for libcore.
* Libraries that aren't tested all mention `test = false` in their `Cargo.toml`
* Crates aren't currently allowed to have dev-dependencies due to
  rust-lang/cargo#860, but we can likely alleviate this restriction once
  workspaces are implemented.

cc #31590

1 files changed, 2 insertions, 0 deletions

diff --git a/src/rustc/test_shim/Cargo.toml b/src/rustc/test_shim/Cargo.toml
index bf576650486..87f2ccd51e8 100644
--- a/src/rustc/test_shim/Cargo.toml
+++ b/src/rustc/test_shim/Cargo.toml
@@ -14,6 +14,8 @@ path = "lib.rs"
 
 [profile.release]
 opt-level = 2
+[profile.bench]
+opt-level = 2
 
 # These options are controlled from our rustc wrapper script, so turn them off
 # here and have them controlled elsewhere.