[test] Output of subprocesses is lost when not using RUST_TEST_NOCAPTURE=1

[Ekleog]

With the following example:

extern crate libc;

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn it_works() {
        match unsafe { libc::fork() } {
            0 => {
                println!("Some stdout");
                unsafe { libc::exit(0) };
            },
            child => {
                let mut status = 0;
                unsafe { libc::waitpid(child, &mut status, 0) };
                println!("Child exited with exit code {}", status);
                panic!("Trigger a failure so that captured output is displayed");
            },
        }
    }
}

Running with cargo test doesn't show the Some stdout text, while running with RUST_TEST_NOCAPTURE=1 cargo test outputs it. Thus some output is lost when not running in RUST_TEST_NOCAPTURE=1 mode, which makes some complex tests less convenient to debug.

[alexcrichton]

I believe this is a dupe of #12309, but thanks for the report!

[Ekleog]

Oh indeed it's a subset of this other bug (which I didn't manage to find, likely because the example uses no-longer-existent std::task, I guess).

Sorry for the noise and thanks!