cargo test -- --format json support?

[bend-n]

Wondering if you'd be willing to expand it, to support cargo test, specifically cargo test --message-format json -- -Zunstable-options --report-time --show-output --format json output.

Ive implemented the test part already, in my cargo-kewl project, so id be willing to make a PR for it.

[oli-obk]

Oh yea, we should totally add it. We'll need a big message about the json format being unstable tho.

[bend-n]

Should I provide a TestCommand too? One that spawns a thread and holds a std::sync::mpsc channel?