testsuite: Update core to std

src/test/run-pass/issue-4735.rs

@@ -9,8 +9,8 @@
 // except according to those terms.
 
 
-use core::cast::transmute;
-use core::libc::c_void;
+use std::cast::transmute;
+use std::libc::c_void;
 
 struct NonCopyable(*c_void);
 

src/test/run-pass/issue-5554.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use core::num::Zero;
+use std::num::Zero;
 
 pub struct X<T> {
   a: T