增强默认值测试并添加新变量

- 测试默认值变量并打印结果
- 新增 pictures_folder 和 wallpaper_folder 变量
- 添加对新变量的测试用例

navi.cheat

@@ -16,4 +16,10 @@ $ user: w
 # 测试默认值
 echo <my_default>
 
-$ my_default: echo "default"
+$ my_default: echo "default"
+
+# Should print /my/pictures/wallpapers
+echo "<wallpaper_folder>"
+
+$ pictures_folder: echo "/my/pictures"
+$ wallpaper_folder: echo "<pictures_folder>/wallpapers"