9227: Add a config setting to disable the 'test' cfg in specified crates r=matklad a=lf-
If you are opening libcore from rust-lang/rust as opposed to e.g.
goto definition from some other crate which would use the sysroot
instance of libcore, a `#![cfg(not(test))]` would previously have made
all the code excluded from the module tree, breaking the editor
experience.
Core does not need to ever be edited with `#[cfg(test)]` enabled,
as the tests are in another crate.
This PR puts in a slight hack that checks for the crate name "core" and
turns off `#[cfg(test)]` for that crate.
Fixes#9203Fixes#9226
Co-authored-by: Jade <software@lfcode.ca>
b48aba0090