You can also write unit tests in the same files as your main code. Eventually you'll want some kind of test suite when your codebase gets big enough, but it works well enough that you don't need to if you don't want to (for a small codebase).

Frameworks like Axum also of course support testing - for instance this crate lets you quickly mock up your server without having to do much: https://docs.rs/axum-test/latest/axum_test/