Testing rexpect app

It would be nice if we could provide an easy way to test rexpect apps. Maybe something like this?

fn main() -> Result<(), Error> {
    let mut p = rexpect::spawn("cat", Some(300))?;

    use_cat(&mut p)?;
    Ok(())
}

fn use_cat(p: &mut PtySession) -> Result<(), Error> {
    p.send_line("I'm a cat")?;
    p.exp_string("I'm a cat\r\n")?;
    Ok(())
}

mod test {
    use super::*;

    #[test]
    fn option1() -> Result<(), Error> {
        use_cat(
            rexpect::testing::FakePtySession()
                .recv("I'm a cat")
                .send("I'm a cat"),
        )?;
        Ok(())
    }

    fn option2() -> Result<(), Error> {
        let mut p = rexpect::testing::FakePtySession();
        std::thread::spawn(|| use_cat(p));
        p.recv("I'm a cat");
        p.send("I'm a cat");
        Ok(())
    }
}

Or any suggestions for a better interface? I think option2 looks nicer in the test case, but I think option1 is overall better, since it doesn't require spinning up a new thread, and needing to sync between the two.
I'm not sure how we could get nice error messages with either approach, but I'm working on option1 at the moment and will see how it turns out.

I haven't seen any crates providing special testing-utilities, is it normally not done? Should we have it behind a feature-flag?

Implementation wise, it looks like we would have to store Box<impl PtyProcess> in PtySession and Box<impl Write> in StreamSession. So that we can keep PtySession without template arguments. This would break the API, but I don't think it should be a problem?