Debugging a recent cargo-outdated bug, it would have been nice not to
wonder whether a rustc version change in GitHub's runner image was a
contributing factor.

This reverts commit dbb973b.

    warning: unexpected `cfg` condition name: `exhaustive`
     --> tests/exhaustive.rs:1:8
      |
    1 | #![cfg(exhaustive)]
      |        ^^^^^^^^^^
      |
      = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
      = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(exhaustive)");` to the top of the `build.rs`
      = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration
      = note: `#[warn(unexpected_cfgs)]` on by default

Resolve unexpected_cfgs warning