Support testing

[mdinger]

A rustbyexample testing example is below. It would be cool if you could click Run or Test in the playpen when there is a test to be run.

fn distance(a: (f32, f32), b: (f32, f32)) -> f32 {
    (
        (b.val0().powi(2) - a.val0().powi(2)) +
        (b.val1().powi(2) - a.val1().powi(2))
    ).sqrt()
}

fn main() {
    println!("If you see this, the tests were not compiled nor ran!");
}

#[test]
fn distance_test() {
    assert!(distance((0f32, 0f32), (1f32, 1f32)) == (2f32).sqrt());
}

[japaric]

It'd be great if the JSON API could be extended to support passing custom flags to the compiler and to the produced binary. That would let us create interactive examples showcasing custom compilation, testing, benchmarking, command line argument parsing, macro expansion (--pretty expanded), etc.

[ranma42]

I have written the changes that should be sufficient to enable testing and benchmarking (and I will PR as soon as I manage to test them). I think this would be pretty convenient as it would make the playpen a place to reproduce behaviours mentioned in Rust PR/issues by simply pasting the code in the playpen. I can see that passing arbitrary command line arguments might be interesting, but I'm afraid of the input validation.
@thestinger Any suggestions on how to tackle this?

[cristicbz]

AFAICT @chris-morgan's PR doesn't deal with benchmarks. Would be really nice if the playpen supported that, allowing people to share microbenches.

[alexcrichton]

I believe this was done awhile back