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/windows/thread.rs

@@ -8,7 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#[cfg(stage0)]
 use prelude::v1::*;
 
 use alloc::boxed::FnBox;
@@ -87,6 +86,7 @@ impl Thread {
 }
 
 pub mod guard {
+    #[cfg(stage0)]
     use prelude::v1::*;
 
     pub unsafe fn current() -> Option<usize> { None }