libstd-rs: Move source directory to library/test

Building libstd-rs from library/std omits proc_macro from the sysroot.
Using library/test causes that to be installed which then allows cargo
to build (https://github.com/meta-rust/meta-rust/issues/266

)

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>