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/process.rs

@@ -815,7 +815,7 @@ mod tests {
     #[cfg(target_os="android")]
     #[test]
     fn test_inherit_env() {
-        use std::env;
+        use env;
 
         let mut result = env_cmd().output().unwrap();
         let output = String::from_utf8(result.stdout).unwrap();