`impl ... for Box<CoreType>` in alloc should not require `#[cfg(not(test))]`

library/alloc/src/ffi/c_str.rs has:

#[cfg(not(test))]
#[stable(feature = "box_from_c_str", since = "1.17.0")]
impl From<&CStr> for Box<CStr> {

As far as I can tell, #[cfg(not(test))] is a workaround for something like https://github.com/rust-lang/rust/issues/87534, as evidenced by error messages like:
note: the crate `alloc` is compiled multiple times, possibly with different configurations

alloc is being compiled a second time as part of tests, and the two compilations are conflicting.

1. This workaround shouldn't be necessary.

2. This should be much better documented, so it doesn't send people on multi-hour debugging adventures when they try to write new trait impls in alloc.

Read library/alloc/src/ffi/c_str.rs and the surrounding #[cfg(not(test))] impl; then trace how alloc is compiled during tests and compare the issue-87534 behavior. Done means the impl no longer needs the workaround and the multi-configuration failure is either resolved or documented clearly for new alloc trait implementations.