How should Rust unit tests be organised?

It’s recommended to place tests in their own module. This module should be a child of the module whose code you want to test (and it can be defined in the same or different file).

#[cfg(test)]
mod tests {
    #[test]
    fn test_some_stuff() {
      // ... test code ...
    }
}

Private methods are not private to child modules, but you still need to import them with use super::some_name;.

By the way, this is all explained in the test organization section (11.3) of the Rust Book.