What is a tree short cut?
You cannot use a tree short cut to level up to achieve a lingot.
A tree shortcut is when you use the test out option for a specific checkpoint. If you pass the test (in other words, you don't lose more than three hearts), all skills leading up to that checkpoint will be marked as completed. If you lose four hearts, you won't pass the test, and if you continuously lose four hearts while testing out multiple times, you'll lose the ability to test out.
As shown in the screenshot I've provided, fourteen uncompleted skills are leading up to this specific checkpoint. Using the test out option would allow me to skip these skills, and they would also be marked as completed.
Once you've progressed past a certain checkpoint, there won't be a test out option anymore, and it will just be labeled as "checkpoint passed".