Warn about `Result`s only used in println!()

A few times now, I've come across tests where a result wasn't verified. Most often, this happens when the result is printed for debugging purposes. See sample code below or have a look at this this real-word example.

While I haven't seen it, I guess this could also be an issue outside of tests or with error!(), warn!(), etc.

I believe it would make since to emit a warning in such cases.

Sample:

some_fn() -> Result<…, …> {
    unimplemented!()
}

#[test]
fn a_test() {
    …
    let result = some_fn();
    println!("result: {:?}", result);   // This compiles with no warning because the `Result` is used by println!()
}

Start from the sample Rust code and investigate how Clippy currently handles unused Result values when they are passed to println!(). Determine the intended scope for println!, error!, warn!, tests, and non-test code; done means the relevant cases emit a warning without incorrectly flagging intentionally handled Results.