Support tests

[dcbishop]

Maybe add a [[test]] to the Cargo manifest format?

Otherwise provide some way for [[bin]] to flag itself as a test and parse the --test flag to rustc.

The first option would seem nicer imho.

[lucab]

Same for benchmarks.

[wycats]

We plan to support tests and benchmarks out of the box. We're still noodling on some ideas.

[alexcrichton]

cargo test is implemented, yay!