Fix difftest Clippy warnings under -D warnings

[uurl]

Closes #367

This fixes existing Clippy warnings in crates/fuzzer/rustlantis/difftest/src/backends.rs so the crate can pass strict Clippy validation with warnings denied.

Changes:

• Return the wait_with_output().expect(...) expression directly instead of binding and returning it.
• Rename private implementation structs LLVM and GCC to Llvm and Gcc.
• Remove an unnecessary borrow in the Miri rustup run ... miri command.

These are mechanical cleanup changes only. The affected structs are private, so this should not change the public API or runtime behavior.

Validation:

cargo fmt
RUST_TEST_THREADS=1 cargo test
cargo clippy --all-targets -- -D warnings