alloc::boxed: enable test

Previously test was disabled due to `#[cfg(test)]` before `mod boxed`.

src/liballoc/lib.rs

@@ -91,6 +91,8 @@ pub mod heap;
 
 #[cfg(not(test))]
 pub mod boxed;
+#[cfg(test)]
+mod boxed_test;
 pub mod arc;
 pub mod rc;