Add xUnit reporting

[jpeach]

Add a --report argument that will write a test result report (possibly in a user-specified format). I don't know of any widely used, well-documented standard format, but xUnit (aka JUnit) and Cucumber both have suitable structured outputs.

I wasn't able to find a spec for cucumber JSON, but there are a few tools out there that generate nice HTML reports from that format.

I don't think that JUnit has a defined spec either, though like Cucumber there are a number fo implementations (e.g. this one in sonobuoy. Everyone mostly seems to just roll their own.

Need to check into how we can push results into the Github PR. There's no built-in Github Action support for test reporting. We could hit the GitHub API directly, or write an adjunct tool to publish the xUnit results.