src/poky: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 (meta-rust/meta-rust#266)

Signed-off-by: Alex Kiernan <[email protected]>
Signed-off-by: Richard Purdie <[email protected]>