json output

[epage]

Tasks

• eRFC: Iterate on and stabilize libtest's programmatic output rfcs#3558
• 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