Add xUnit compatible reporter

[erik]

Would be useful to have a reporter which dumps results in the standard (ish) XML format used by JUnit etc. so that this can integrate more easily with existing tooling.

It's obviously much more geared toward test runs, so we'll need to see if lint output makes any kind of sense here.

https://xunit.net/docs/format-xml-v2

[erik]

Haven't felt the need for this, will revisit if someone requests it.