feat: expose rust_test_suite

[finn-ball]

Summary

• expose the upstream rust_test_suite macro through a public rules_rs wrapper
• add the generated bzl_library target
• exercise the wrapper with two generated Rust test targets

Why

rules_rs recommends loading Rust rules through its facade, but users currently need to load rust_test_suite directly from @rules_rust. This adds API parity with the pinned upstream ruleset and makes migration and consistent loading simpler.

Validation

• buildifier rs/rust_test_suite.bzl rs/BUILD.bazel test/BUILD.bazel
• bazel test //:rust_test_suite from the test workspace (2 tests passed)
• git diff --cached --check

[dzbarsky]

Thanks for the PR, my reasoning on this is #193 (comment). Apologies for not having time to actually do what I intend yet, this one has been a bit low on my priority list given the easy workaround :) Curious to hear if this would solve it for you?