"expensive_tests" feature fails to build from source

[BenWiederhake]

$ cargo +nightly --version
cargo 1.81.0-nightly (4ed7bee47 2024-06-25)
$ cargo +nightly build --all-features
[… some other warnings …]
   Compiling coreutils v0.0.27 (/home/user/workspace/coreutils-rs)
error[E0433]: failed to resolve: use of undeclared crate or module `expensive_tests`
   --> /home/user/workspace/coreutils-rs/target/debug/build/coreutils-95c1781d49038d86/out/uutils_map.rs:182:30
    |
182 |         ("expensive_tests", (expensive_tests::uumain, expensive_tests::uu_app)),
    |                              ^^^^^^^^^^^^^^^ use of undeclared crate or module `expensive_tests`

error[E0433]: failed to resolve: use of undeclared crate or module `expensive_tests`
   --> /home/user/workspace/coreutils-rs/target/debug/build/coreutils-95c1781d49038d86/out/uutils_map.rs:182:55
    |
182 |         ("expensive_tests", (expensive_tests::uumain, expensive_tests::uu_app)),
    |                                                       ^^^^^^^^^^^^^^^ use of undeclared crate or module `expensive_tests`

For more information about this error, try `rustc --explain E0433`.
error: could not compile `coreutils` (bin "coreutils") due to 2 previous errors
warning: build failed, waiting for other jobs to finish...
error: could not compile `coreutils` (bin "uudoc") due to 2 previous errors
[$? = 101]

(Also happens with stable and 1.70, but I wanted to demonstrate that it's not a 1.70-only issue.)

This seems related to #6498, so CC @cakebaker

[cakebaker]

Wondering why this issue doesn't show up in our CI.