feat(core): add colocated *_test.rs and test_*.rs to Rust group

Measurement across 10 large public Rust repos showed the tests.rs-only
group carries almost no weight — the majority of file-glob hits come
from workspaces that colocate a `foo_test.rs` next to `foo.rs` rather
than using extracted `tests.rs` modules. In paritytech/polkadot-sdk
this shape accounts for 232 files and a 15% reduction on the analysed
budget; in rust-lang/rust it accounts for 253 files (3% reduction on
the truncated tree, i.e. a lower bound).

Add:
  **/test_*.rs   — pytest / unittest style (less common in Rust)
  **/*_test.rs   — dominant convention in Rust workspace monorepos

Kept as separate globs from `**/tests.rs` so users on projects that
follow the Rust Book pattern (single sibling tests.rs per module)
aren't forced to also strip a naming shape they never use.