When starting a new software product your speed of development is extraordinary. It is so painful, seemingly counter intuitive, to slow that process down by spending time writing tests that will automatically execute every time you deploy your software, which should happen as often as possible, hopefully many times per day if you are creating shippable chunks that can be deployed as they are done.
If you don't spend the time up front on test coverage and continuous deployment processes the rate of development for your project will get slower and slower as your code base gets bigger, and more people join your team. In other words you will pay the price of not doing the up front work for the remaining life of your product. DON"T DO IT. Write your tests, then your code, and perfect your continuous deployment as you go so that your rate of development is constant throughout the lifecycle of your project. Slow and steady wins the race.