Fix compile-fail arc/sync tests

Author: bblum

src/test/bench/graph500-bfs.rs

@@ -5,6 +5,7 @@ An implementation of the Graph500 Breadth First Search problem in Rust.
 */
 
 use std;
+import std::arc;
 import std::time;
 import std::map;
 import std::map::map;

src/test/compile-fail/no-capture-arc.rs

@@ -1,6 +1,7 @@
 // error-pattern: copying a noncopyable value
 
 use std;
+import std::arc;
 import comm::*;
 
 fn main() {

src/test/compile-fail/no-reuse-move-arc.rs

@@ -1,4 +1,5 @@
 use std;
+import std::arc;
 import comm::*;
 
 fn main() {

src/test/compile-fail/sync-cond-shouldnt-escape.rs

@@ -1,4 +1,7 @@
 // error-pattern: reference is not valid outside of its lifetime
+use std;
+import std::sync;
+
 fn main() {
     let m = ~sync::mutex();
     let mut cond = none;