Add marketplace test results parser

This PR introduces the parser which reads the results.xml files from each job and aggregates the results into a json file that the marketplace tests dashboard can consume.

This is all working locally for me, and should work when run on Jenkins as well. My idea is to set it up so that this will be run automatically after every marketplace job, which will keep the file always up to date.

Right now this just generates the file locally. We'll also need to get it to the gh-pages branch so it can be used. I was thinking I'd use a similar approach to what is being done with the xfails dashboard, where we simply use Git to push the new file to the repo. I have a couple of questions about this, which I want to discuss with @retornam.

Requesting review from @retornam and any other interested parties.