feat: re-export rust_test_suite

[pdeva]

Summary

• re-export upstream rust_test_suite from the rules_rs Rust test façade
• add an integration smoke test with two test roots and a shared source

Why

rules_rust already provides rust_test_suite for Cargo-style integration
tests, but rules_rs only exposed rust_test. Consumers therefore had to
either load from the generated @rules_rust repository directly or maintain
their own equivalent macro.

Exposing the suite alongside rust_test keeps consumers on the @rules_rs
public API.

Validation

• bazelisk build //rs:rust_test
• cd test && bazelisk test //:rust_test_suite_smoke
• buildifier check on the changed Starlark files
• rustfmt --check on the new Rust test sources
• git diff --check

[dzbarsky]

@pdeva Thanks for sending this up! I've been thinking we should actually expose a simplified/cleaned-up version of this rust_crate macro - the idea being that we should be generating not only the unit tests like rust_test_suite, but also the per-binary test targets, like Cargo does. As part of that I can decompose to some helper macros so the piece can be used independently, that can cover rust_unit_test_suite (note unit in the name) as you are doing here. WDYT of that approach?

[pdeva]

@dzbarsky i like your approach 👍