Fix test compile-fail/task-rng-isnt-sendable.rs.

src/test/compile-fail/task-rng-isnt-sendable.rs

@@ -10,10 +10,10 @@
 
 // ensure that the ThreadRng isn't/doesn't become accidentally sendable.
 
-use std::rand; //~ ERROR: module `rand` is private
+use std::__rand::ThreadRng;
 
 fn test_send<S: Send>() {}
 
 pub fn main() {
-    test_send::<rand::ThreadRng>();
+    test_send::<ThreadRng>(); //~ ERROR std::marker::Send` is not satisfied
 }