syntax: Don't assume std exists for tests

This commit removes the injection of `std::env::args()` from `--test` expanded
code, relying on the test runner itself to call this funciton. This is more
hygienic because we can't assume that `std` exists at the top layer all the
time, and it meaks the injected test module entirely self contained.

src/libstd/sys/unix/thread.rs

@@ -166,6 +166,7 @@ impl Drop for Thread {
           not(target_os = "netbsd"),
           not(target_os = "openbsd")))]
 pub mod guard {
+    #[cfg(stage0)]
     use prelude::v1::*;
 
     pub unsafe fn current() -> Option<usize> { None }