test: add open is builtin command test

src/builtins/mod.rs

@@ -74,6 +74,11 @@ mod tests {
         assert_eq!(is_builtin("pwd"), true);
     }
 
+    #[test]
+    fn test_is_builtin_open() {
+        assert_eq!(is_builtin("open"), true);
+    }
+
     #[test]
     fn test_is_builtin_change_prompt() {
         assert_eq!(is_builtin("change-prompt"), true);