Add a test for sort_by_key

Author: varkor

src/liballoc/tests/slice.rs

@@ -425,6 +425,11 @@ fn test_sort() {
                 v.sort_by(|a, b| b.cmp(a));
                 assert!(v.windows(2).all(|w| w[0] >= w[1]));
 
+                // Sort in lexicographic order.
+                let mut v = orig.clone();
+                v.sort_by_key(|x| x.to_string());
+                assert!(v.windows(2).all(|w| w[0].to_string() <= w[1].to_string()));
+
                 // Sort with many pre-sorted runs.
                 let mut v = orig.clone();
                 v.sort();