Improve test report details

[jasonmp85]

It sure would be nice to have things like error logs, differences in test output, etc., right in our UI. I think CircleCI should be able to do this. Their documentation on Collecting Test Metadata gives a high-level overview, but their reliance on the JUnit format (which is documented only in a few ancient XSD files floating around the web) means we'll need to use other implementations as inspiration.

Fortunately, CircleCI themselves forked assumed maintainership of minitest, so we can look at precisely how they generate test metadata after a run.

The RSpec JUnit formatter's xml_dump method could be another good place to look.

Basically, look at what they do and do something similar for the properties we care about in our tests.