test: add ls is builting command test

src/builtins/mod.rs

@@ -76,6 +76,11 @@ mod tests {
         assert_eq!(is_builtin("pwd"), true);
     }
 
+    #[test]
+    fn test_is_builtin_ls() {
+        assert_eq!(is_builtin("ls"), true);
+    }
+
     #[test]
     fn test_is_builtin_open() {
         assert_eq!(is_builtin("open"), true);