Disable `explicit_write` in test code?

[smoelius]

Would you consider a PR to disabe the explicit_write lint in test code?

An explicit write allows you to circumvent libtest's stdout/stderr capture, which you might to do to emit warnings, for example.

The below test can be used to verify this (playground):

#[test]
fn test() {
    use std::io::Write;
    writeln!(std::io::stderr(), "I am an explicit write.").unwrap();
    eprintln!("I am not an explicit write.");
}