build: convert V8 test JSON to JUnit XML
This introduces some code to convert from V8's test JSON output to JUnit XML. We need this because V8's latest refactor of their test runner has made it difficult to float our JUnit reporter patch on top (see the referenced issue).
I also think that there needs to be the same changes to vcbuild.bat, but I don't know how to do/test those yet. I can create a Windows VM and test it if we decide to go with this approach.