Support libtest's format=json unstable feature (#194, #203, #218)

[ilslv]

Resolves #194, #203, #218

Synopsis

By default CLion (and other JB IDEs) pass -- --format=json -Z unstable-options --show-output set of flags, which enables libtest (default rust test harness) JSON formatting. Currently this fails the CLI, because those flags are unknown.

Solution

Support JSON formatting.

Default output:

With feature cargo-json and -- --format=json flag:

As libtest's JSON output first line is:

{"type":"suite","event":"started","test_count":41}

We should somehow know number of test cases. Because of the event streaming nature of this library, to get number of test cases a new event has been added: event::Cucumber::ParsingFinished, which is emitted by runner::Basic right after all Features are parsed. There are no guarantees on when this event will be emitted, but it should happen exactly once.

Checklist

• Created PR:
  • In draft mode
  • Name contains Draft: prefix
  • Name contains issue reference
  • Has assignee
• Documentation is updated (if required)
• Tests are updated (if required)
• Changes conform code style
• CHANGELOG entry is added (if required)
• FCM (final commit message) is posted
  • and approved
• Review is completed and changes are approved
• Before merge:
  • Milestone is set
  • PR's name and description are correct and up-to-date
  • Draft: prefix is removed
  • All temporary labels are removed

[ilslv]

FCM

Support Rust `libtest`'s `--format=json` unstable feature to integrate with IntelliJ Rust pugin (#220, #194, #203, #218)

- add `libtest` Cargo feature
- impl `writer::Libtest`
- describe IntelliJ Rust integration in Book

Additionally:
- rework `writer::Failure`/`writer::discard::Failure` as `writer::Stats`/`writer::discard::Stats`
- add `writer::Or` to alternate between 2 `Writer`s basing on predicate
- add `FeatureExt::count_steps()` method

[tyranron]

@ilslv nice job overall 👍

Though, some stuff is not quite clear, and some needs polishing.

[ilslv]

nightly ICE has already been reported.

[tyranron]

@ilslv thanks!