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/thread/local.rs

@@ -329,6 +329,7 @@ mod imp {
     // Due to rust-lang/rust#18804, make sure this is not generic!
     #[cfg(target_os = "linux")]
     unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
+        use prelude::v1::*;
         use mem;
         use libc;
         use sys_common::thread_local as os;