Make CI tests locally replicalbe for anyone cloning this repo

[tnballo]

The current test infrastructure uses large files (QCOWs, replays) stored locally on the CI server. That's good because running the tests doesn't waste time downloading these files, but ideally anyone cloning this repo should be able to run the same tests. Scripts should support a flag that downloads required files from https://panda-re.mit.edu/ when run locally.