Un-xfail test for #3521; it works now

Closes #3521

Author: catamorphism

src/test/run-pass/issue-3521.rs renamed to src/test/compile-fail/issue-3521.rs

@@ -1,9 +1,8 @@
-// xfail-test
 fn main() {
     let foo = 100;
 
     enum Stuff {
-        Bar = foo
+        Bar = foo //~ ERROR attempt to use a non-constant value in a constant
     }
 
     log(error, Bar);