added fp test against negative .step_by(_)

tests/compile-fail/for_loop.rs

@@ -1,4 +1,4 @@
-#![feature(plugin)]
+#![feature(plugin, step_by)]
 #![plugin(clippy)]
 
 use std::collections::*;
@@ -71,6 +71,10 @@ fn main() {
         println!("{}", i);
     }
 
+    for i in (10..8).step_by(-1) {
+        println!("{}", i);
+    }
+
     let x = 42;
     for i in x..10 { // no error, not constant-foldable
         println!("{}", i);