Make tokio unstable opt in

[mcches]

We use unhandled_panic to forward host software panics
as test failures.

Consider the test:

#[test]
fn panics() -> turmoil::Result {
    let mut sim = turmoil::Builder::new().build();

    sim.host("host", || async {
        tokio::spawn(async { panic!("boom!") });

        future::pending().await
    });

    sim.client("client", async { Ok(()) });

    sim.run()
}

With tokio_unstable the test fails, which can be useful for
uncovering issues early.

Forcing this is a bit invasive for consumers of the crate, so
instead we make it enabled conditionally.