Initial JUnit report support.

[jvprat]

I've started working on JUnit XML report support. Does it look like something you'd be interested to merge? It still needs some work and a lot of testing, but the code is simple enough that others could be interested in it. For now I've just added the existing data to the reports. If we wanted to add test execution time and stdout/stderr it would need some more intrusive modifications on the run code.