Give a better error message when running cargo test on libstd itself

[jyn514]

Right now, you get really terrible errors like this:

$ cargo test
error[E0277]: the size for values of type `()` cannot be known at compilation time
   --> /home/joshua/.local/lib/cargo/registry/src/github.com-1ecc6299db9ec823/getrandom-0.1.14/src/use_file.rs:30:44
    |
30  | pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
    |                                            ^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `()`

162 times. It would be more helpful to instead say 'use x.py test instead'.

[jyn514]

Fixed by #94796.