Include ignore message in libtest output

As an example:

    #[test]
    #[ignore = "not yet implemented"]
    fn test_ignored() {
        ...
    }

Will now render as:

    running 2 tests
    test tests::test_ignored ... ignored, not yet implemented

    test result: ok. 1 passed; 0 failed; 1 ignored; 0 measured; 0 filtered out; finished in 0.00s

compiler/rustc_expand/src/build.rs

@@ -329,6 +329,10 @@ impl<'a> ExtCtxt<'a> {
         self.expr_call_global(sp, some, vec![expr])
     }
 
+    pub fn expr_none(&self, sp: Span) -> P<ast::Expr> {
+        let none = self.std_path(&[sym::option, sym::Option, sym::None]);
+        self.expr_path(self.path_global(sp, none))
+    }
     pub fn expr_tuple(&self, sp: Span, exprs: Vec<P<ast::Expr>>) -> P<ast::Expr> {
         self.expr(sp, ast::ExprKind::Tup(exprs))
     }