json output

[epage]

Tasks

• [x] rust-lang/rfcs#3558
• [x] Move https://github.com/epage/pytest-rs/ to rust-lang, see https://github.com/rust-lang/libtest-next
• [ ] Finish libtest-next experiment: https://github.com/rust-lang/libtest-next/milestone/1
• [ ] Upstream in libtest
• [ ] Experiment with cargo test ui on top of this
• [ ] Stabilize