test: speed up compile-test suite

[tisonkun]

Summary

• register compile-pass fixtures through the standard Rust module tree: tests/ui.rs -> tests/ok/mod.rs -> one file per case, with no repeated #[path] attributes
• run the 19 pass cases as native Cargo tests so the test harness can schedule them independently, while keeping trybuild focused on the 18 compile-fail snapshots it can batch
• fix case-local warnings and Clippy findings with real assertions and used values; remove broad lint suppression and stale comments

Benchmarks

Measured on macOS arm64 with Rust 1.91.0:

• clean target: 32.81s -> 17.60s (46% faster)
• warm target: 8.97s -> 0.36s (96% faster)

Verification

• cargo +nightly fmt --all -- --check
• cargo test -p tests-build
• cargo +1.91.0 nextest run -p tests-build
• cargo +stable nextest run -p tests-build
• cargo +nightly nextest run -p tests-build
• cargo +nightly clippy --all-targets --all-features -- --deny warnings