Support a proper junit reporter

[bitliner]

Feature

Add a JUnit reporter and allow saving the output to a file.

Motivation

JUnit reporting format is a standard used by many CI/CD tools, like gitlab, CircleCI, etc.

If you do CI/CD, JUnit reporting is great to improve user experience while maintaining software packages and related tests.

In addition, NodeJS test runner supports very well out-of-the-box the junit reporter and concurrency at the same time. Since ava lacks proper support for junit reporting, it risks to become outdated and lose effectiveness.

The problem

Ava does not support JUnit reporter.

Even though it supports tap reporters, and you can use tap parser to transform the tap output into a junit reporter, however ava does not provide full information to the tap reporter and therefore does not allow to build a proper junit report.

As example, time duration is missing.

There is an issue open to add duration, however doing it only on --serial would be not enough to build a proper junit report.