Your whole premise is based on the the idea that your startup is going to fail. In that case why write any code at all?
If your startup doesn't fail then you've really made things extra hard on yourself. One of my favorite things about working on my startup is getting to learn and apply good practices.
It's just as easy to write good code as it is to write bad code. I find that TDD speeds me up not slows me down. It's not as much overhead as people seem to suggest.
You can write new code and then keep running the program to see if it works. Or you could write the code and keep running a test until it works. The test is typically faster for this because you don't have to deal with the rest of the program which is unrelated to what you're working on. Once you have working unit tests the implementation is typically trivial.
In practice, I don't write tests for everything but I code in a way that means I could test everything if I wanted to.