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/lib.rs

@@ -414,11 +414,3 @@ pub mod __rand {
 // the rustdoc documentation for primitive types. Using `include!`
 // because rustdoc only looks for these modules at the crate level.
 include!("primitive_docs.rs");
-
-// The expansion of --test has a few references to `::std::$foo` so this module
-// is necessary to get things to compile.
-#[cfg(test)]
-mod std {
-    pub use option;
-    pub use realstd::env;
-}